If you want to swim in Monster Hunter Stories 2, you will need to get a monster that can swim, such as the Royal Ludroth, Zamtrios, and Lagiacrus. All three of these monsters have a unique ability that allows them to swim across areas of water.
However, you won't be able to get two of them until later in the game. Royal Ludroth, on the other hand, can be obtained via soggy leviathan egg pretty early on in chapter two if you know where to look. In short, you will need to check various monster dens, until you find one that has Royal Ludroth or one of the other monsters that have swim as an rider skill.
When you find them, you will need to force them to retreat using a paint bomb when they ideally have low health and by breaking their parts. Each monster also has a list of weaknesses that can be used to defeat them as well.
That said, locate one of the monsters and hatch their eggs, and you will be able to swim across areas blocked of water to get what are often more monster dens and sometimes even chests. But if you have trouble finding a monster, we have a guide linked above that will help you find the Royal Ludroth.
Check out some of our other Monster Hunter Story 2 guides!