It's alright. I've got you - I won't let anything happen to you. [He didn't swim too much in Camelot, not being as close to the sea, but he's strong enough to swim while holding her, regardless, and focuses on keeping her above water - although jfc it's true that her dress is rather heavy]
Just hold onto me, alright? Let me know if you're having trouble breathing. [And very slowly starts kicking off towards shallower water, mindful not to go too quickly and risk getting more water in her mouth]