Why choose a seaplane for crew rotation at sea, supply of spares to ships, long-range search and rescue and surveillance?
The US-2 is an amphibious airplane not a pure seaplane, it can land on water and land. MTOW is a bit higher on land as well.

Purely logistics alone means that a single craft type is better.

It is better to have a jack of all trades than a bunch of specialists when building out your fleet unless you have immediate need for those specialists.

Each aircraft type that gets added to the fleet has its own maintenance handbook, maintenance schedule, set of spares, their own type rating, etc.

Helicopters have smaller tanks and can't stay in the air as long or get out as far away from base. They also cannot carry as much stuff as the US-2 can. They would be handy for search and rescue and supplying helipad-equipped vessels. However not all ships will have a helipad.

Jets cannot land out at see so they can only do the search part of search and rescue. They cannot do any supplying out at sea that doesn't involve putting the packages on a parachute.