Re: gas vent cap
 

Actually, I just had this happen to me, and I've been meaning to post the info here.

If your fuel/holding/waste tank vent looks like this:
http://www.marineeast.com/me-images/s_img_std/956.jpg

and you've damaged just the screw-on cap, you can purchase a replacement from:

MarineEast -- Products

Part number is 3055 (or 3055W if you want white) and they're $1.60 each.