Hi, without being able to see your parameters, it is hard to know what you have set up that might be going wrong.
First thing to check is to make sure the player has access to swimming. You can either start a new game with the "Swim Access" parameter under "Initial Settings" set to true, or you can use the plugin command "Change Swim Access" and set it to "true".
Some less common issues you may be running into:
If you have set the "swim key" to a capital letter, you will need to hit shift+that letter.
If you have a Swim Enter or Exit region set, the player will only be able to enter/exit swimming while standing ON those regions.
Check that the boat can pass the tiles you are trying to swim on. Swimming gives the player boat passability, and the plugin will check if the boat can move on the tile you are trying to swim on. Depending on how your tileset is set up, your water tiles may not be boat passable.