The two sides of the Navy/Marine Corps amphibious coin — ship to shore via traditional watercraft and via aircraft such as the newer tilt-rotor Osprey — are a big focus of Rim of the Pacific exercises, and a growing interest for partner nations in the region.

That importance is highlighted in the high-tech ships here for RIMPAC.

The stealthy USS San Diego is among the most advanced amphibious ships ever built. The $3 billion USS America is the size of a World War II aircraft carrier. And HMAS Canberra, the largest ship Australia has ever built, can land more than 1,000 personnel by helicopter and watercraft.

The U.S. amphibious force is “flexible, scalable, and is rapidly responsive,” the Center for Strategic and International Studies said in a June report. “As a result, the amphibious force is frequently called on during the initial phase of a crisis response.”

The modern focus includes humanitarian assistance in natural disasters, but the goal is the same: getting forces inland from the sea without relying on established infrastructure.

“With that amphibious capability, I can put a force right next to a bad situation like that,” said Marine Brig. Gen. Ray Descheneaux, snapping his fingers, “and respond to whatever’s coming our way.” Descheneaux commands the Fleet Marine Forces for RIMPAC.

Japan and Australia are working to establish standing amphibious capability, according to CSIS. South Korea is expanding its ship capacity, and India, the Philippines and Singapore want stronger amphibious forces.

“I think amphibious operations are the way the future is going,” said Maj. Richard Thapthimthong, operations officer for the 2nd Battalion, Royal Australian Regiment, which makes up the core of the country’s amphibious landing force.

A platoon of up to 30 2nd Battalion soldiers was practicing inflatable boat landings in the surf Friday at the Kaneohe Bay Marine Corps base next to U.S. Marines from Combat Assault Company, who were heading out to sea and back in their 26-ton tanklike amphibious assault vehicles.

Japanese soldiers were also training with small boats in surf that sometimes bounced the black inflatable craft into the air.

“You look at the geographic layout of the world,” Thapthimthong said. “The vast majority of the population is along the coastline … so I think to master the ability to maneuver from the sea, to bring aid from the sea, to help provide hospital care from the sea, I think that’s going to be the way the future goes — especially within the Pacific region, and Australia is a very key player there.”

As part of greater interoperability with Australia’s forces, U.S. Marine amphibious assault vehicles will drive into the well deck of the Canberra, and Osprey tilt-rotor aircraft will land for the first time on the Australian ship’s deck, officials said.

The at-sea portion of RIMPAC, which starts this week, will also feature dual philosophies of getting to shore — by sea and by air.

The 844-foot America, commissioned in late 2014, doesn’t have a rear well deck for watercraft to enter and exit, a common feature on ships in the amphibious fleet, otherwise known as the Gator Navy.

At the start of the United States’ entry into World War II on Dec. 7, 1941, the Navy had no oceangoing ships capable of discharging a cargo of big tanks or other heavy equipment onto a beach without the aid of piers or cranes, according to the service.

That all changed, quickly, and in that ongoing evolution the America and the next ship in the class, the Tripoli, are an experiment in what the Navy calls an “aviation-centric” design. No amphibious assault vehicles, big cargo-carrying hovercraft or other landing craft means more room for Ospreys, helicopters and, coming soon, F-35B jump jets.

Capt. Michael Wayne Baze, commander of the America, said the ship has about 40 percent more volume in aviation capacity in terms of square footage. The configuration is suited to the longer-range Ospreys and F-35B Joint Strike Fighters, allowing operations farther inland, but the Navy hasn’t given up on the well deck design, either.

The next ship after the Tripoli in the class will reintroduce a well deck, with the need remaining to transport heavy equipment ashore like trucks and tanks that can’t be airlifted by helicopters. Also to be upgraded are the amphibious assault vehicles, tracked armored vehicles that are used by Combat Assault Company at Kaneohe Bay.

Thirteen of the AAVs were on the beach at Pyramid Rock on Friday for oceangoing practice ahead of duty onboard the USS San Diego and beach landings later in the exercise.

The AAVs, which carry 21 combat-loaded Marines and three crew, can bob underwater momentarily when they drive off a ship, said Cpl. Joseph Hunt, 20, a crew chief who has been with the AAV company for more than two years.

Hunt said driving up to 3 miles at sea in a tanklike vehicle “is definitely an acquired taste,” adding, “The first time out it’s always a little scary for everyone, just a new experience. But once you get out there, it starts becoming a good time.”

Twin long hatches open and close on the top, and with the hatches closed at sea, seawater “will be dripping in. Sometimes you’ll get a little stream come through,” he said.

Cpl. Laurence Jensen, 20, an AAV mechanic, said since the war in landlocked Afghanistan, the vehicles “have definitely gotten a little forgotten about.” But with the pivot to the Pacific and renewed emphasis on amphibious operations, they are “getting a name for themselves again.”

The Marines created a Marine Air-Ground Task Force with more than 3,000 Marines, sailors and soldiers for RIMPAC. Capt. Tim Irish, a Marine Corps spokesman, said the Marines and partner nations have been busy since last week practicing getting on and off Ospreys and CH-53E helicopters and securing objectives, live-firing weapons, and training at the shoreline.

Kaneohe Bay’s 2nd Battalion, 3rd Marines, has about 1,000 Marines participating, with Australia contributing 300 to 400 soldiers.

Irish said the actual amphibious scenario the three ships will face at sea isn’t known yet, but it involves the Hawaiian Islands playing fictitious countries “and a range of military operations that a maritime force would respond to.”

“So anything from humanitarian assistance to noncombatant evacuation operations to more on the high end of … forcible entry from the sea, flying forces off a ship,” he said.