I've been welding for 40+ years now and have a great deal of experience TIG and MIG welding aluminum. However my experience has been on heavier sheet aluminum 11ga. 1/8" and heavier but have considerable experience welding dirty aluminum castings. I doubt I'd even attempt the repairs you're needing with my experience level. Now if you had a broken skeg that's a different story, bring it over and I'll weld it up! If it were thin gauge stainless I'd take the job, I do have considerable experience with thin gauge stainless.

Being located where you're at doesn't lend to much options for repairs. Does the boat have insurance on it? If so contact your insurance company perhaps they have someone in their database who is qualified to make the repair for you. Seems like you're between a rock and a hard place, I wish you the best of luck getting the boat repaired.