I do like this mount a ton, but it's strange that it's actually slower than a player's base swim speed when underwater, and the boost barely does anything (like, I had to stare directly at the speedometer to realize it did anything at all. Please make the boost skill actually do something underwater, right now it's a waste). A dismounted player shouldn't be able to straight up outpace a mounted player. This combined with the slam effect not working on enemies unless they're right on the sea floor means there's no reason to use this mount underwater right now.
I get that underwater combat isn't the crux of GW2 gameplay, but given how much effort went into this it feels like a waste.
Otherwise I had a great deal of fun rampaging around with this guy.