Ok.. i stop lurking now
I liked whatyou've done so far..
I dont know anything about gamemaker but it loks like you could build a decent shmup in it if you work at it..
Personally I'd advise working on the gameplay first and the graphics later.. Because if you are programmng it yourself you will have to implement all the game play ideas.. whereas it is possible to get someone else to do the gfx later. and a dull game with great gfx isnt that hot anyway..
But i reckon you have got a nice idea with the underwater theme!!
I'd try and exploit it more.. say having bubble gun as a 2nd weapon that you can fire to capture enemies with, once youve lined up a few in bubbles you could pop em with the first weapon and rack up some extra score (damn.. thats a good idea.. maybe i do it myself...
And maybe try and do more stuff graphically with the water effects.. like background caustics or deep water rocky stuff. coral reefs etc
check out 'in the hunt' or especially the water level in 'dragon blaze' which are both water shmups with great graphics..
Keep it up!
p.s. i like the spiky things that shoot off the side walls..