Force Recon was created in the 1950s. Its mission, then and now, was to go deep behind enemy lines in order to gather reconnaissance on the enemy (hence the name Force Recon). They are similar to the Navy SEALs in that they are heavily trained in airborne and combat diving techniques in order to allow them to bypass traditional land defense systems.
The water, and swimming, is a heavy component of being in Force Recon (they are Marines after all, we operate on the sea). The difference between them is that the SEALs’ purpose is to kill the enemy, while Force Recon’s main purpose is simply to gather intelligence. Force Recon’s mission is considered a success if absolutely no shots are fired. Marines in Force Recon are absolutely trained in direct-action missions, and are capable of performing in combat situations, but that is not their primary role.