get a bowfishing kit, such as a slotted reciever from AMS or Muzzy. basically it is a big fiberglass arrow, with a string tied to it. the other end of the string is tied to a big buoy. shoot the gator, he takes out line and buoy, you follow the buoy fight him, get him up to the boat, stick another arrow in him if possible. kill him however you feel fit.

if you shoot him with a regular arrow and broadhead, he would just swim under and youd never see him again...

definately gotta go with the bowfishing equip. go to muzzy.com and they even have kits for like 200 bucks that have everything you need.....good luck.