I guess 2 extra promos, way too overpowered too, considering at 5 exp it would have 4 (!!!) promos. and 10 exp 5 promos...no, please, it's already strong enough being a amphibious infantry and getting a lot of power against armored units (the same ammount my Tiger gets?). maybe a smaller bonus, like instead of 2 promos and +50% against armor, +25% defending against armor and +25% in river attack? (just a example)
But seriously, the bonus against armor is overpowered...it already can attack nicely from ships or across rivers. i don't remember the marine power btw, so i can't say it is that overpowered.