Swimming is a valid and useful tactic, however it should be a high risk tactic.
As it stands currently, trying to use ranged against anyone in water is much more difficult than on land/walls as the engine doesn't seem to register hitboxes that well. This results in someone in light/medium gear effectively able to block any ranged attack while in the water with little/no counter to it.
If it was to remain, then attacking or blocking while swimming should carry significantly more stamina drain than is currently implemented in game. This would still allow it as a tactic, but it would balance it from being abused due to poor hitbox design.