MArcomage

I think this card is over the top for a common. 15 attack with full Aqua hand (13 average) and 4G/4R mixed cost plus Aqua keyword - it's kinda too much. This card is better than any of attack commons except Death wave and Angry mob (under certain circumstances).
I suggest to lower its attack from 7 + N to 4 + N.

Fireball can also do 14 damage + 5 straight tower attack under certain circumstances.
Angry mob can easily do more than 12 damage and it's 0-cost now.

Maybe change it's cost to 8R.

I haven't used it, but the way I look at it, if your deck is such that you'll have 8 Aqua on hand, the cost (and timing, since you want to play it after a non-Common Aqua) is currently appropriately prohibitive.

If a change is to be made, I'd just cap N at 6.

This common is covering the main Aqua cardset weakness - lack of attacking commons. I mean, Sea serpent is much more specific, and others aren't even near in terms of attack. Aqua uncommons and rares are expensive, there is only one Aqua summoner and not enough Aqua cards to fill all uncommon and rare slots, so you'll have to play with 7-8 Aqua commons in hand. And in this case you could break enemy resources, steal it, defend yourself or raise stock, but you couldn't attack seriously until an uncommon draw.
Giving a common like this to Aqua is like giving Brigand keyword to Angry mob.

I would propose two variants:

A - cap N to 6

B - lower base attack to 5 from 7

B, of course.