Method 1. Half submerged Some girls, like Kongou for example, have a boat attached to them so they're meant to be half submerged. But then some girls have equipment on their lower bodies so this wouldn't work.

Method 2. Gliding on the water surface This method of movement disregards the design elements on their upper bodies. Some girls wear shoes specifically designed for this. I think this is the most reasonable one.

Method 3. Appear like stands spirits or fairies My personal favorite.

Method 4. Don't actually fight in battles If they didn't need to fight then they don't need to enter water either.

I've come to buy black tea.