Lets not say this is a real bug but Since seaplants were added and swim animations were also added Minecraft became much better.But there is a issue that get caused of minecrafts Third Person mechanism.As you know when you touch/go inside a block when you are in Third Person Wiew the mechanism makes you see inside of your character. Since there are so much Kelps in sea you always get inside block and it makes it disgusting to swim with Third Person in sea.