Amphibious Assault Cavalier [PF]

[The Oni]

So, I'm presently working on a very strange build for a Merfolk Luring Cavalier/Mysterious Stranger Gunslinger/Arcane Duelist Bard. The idea is a singing, rifle-wielding, absurdly over-the-top assassin, in the vein of a fantasy-themed No More Heroes. Strictly speaking, this build doesn't have to be *legal,* because I'm a GM and I'll likely as not be using her to terrorize my players - but I'd prefer to avoid needless handwavium and stick as close to the rules as possible.

Now my question is - weapons, for a mount. Assuming her initial attack goes wrong, she has a large mount that she straps cannons to; this would more or less be used like cannons on a ship, with the cannons pointing off the side and attacking in a firing line. I intend to use the stats for the Culverin. I figure with an Orca-statted whale I can probably fit four or five cannons on each side, but I want to be sure I'm using all of my options. Assuming anything with Animal Companion stats is legal as a mount, is it possible that I could achieve:

A Large amphibious mount (before Level 5)
A Huge mount (before Level 5)
Either of the above

[Nihilarian]

The only way to get a huge mount is to take the Mammoth Rider PrC, I think.

As to a large mount before level 5, I don't think so.

Edit: this is speaking from an animal companion point of view. You could easily make the merfolk a musketeer archetype to get rid of the animal companion and give her a pet whale.