massive corrosion

Is the hole directly under the fuel tank, if not he may get away with welding a patch on the outside.
It is hard to discriminate but looks like electrolysis so you won't just be able to clean it up and fill the hole.

 Horible, dirty contaminated ali = prick of a job. You would be wanting to look at the cause, could be a horror story on the inside.

As above would want to ascertain if its electrolysis  from something such as a dropped swivel etc as even if you repair it it's only going to come back/worsen

 Based on my unfortunate experience, you HAVE to pull it apart to get to the root cause and fix it properly.

 It might not be as bad as some are saying but either way it needs welding so like it or not the fuel tank will have to come out.

i just had some repair work done by Wakemaker marine in Henderson take it down and see Mark he will sort you out and not just do a patch job like some guys will that probably won't fix the bigger problem

if it were my hull I would be opening up the deck to have a good look at what's going on in there 

 Looks like mine, electrolis, see Jim at marineline wangara, are they all pitted holes as well

nobody in their right mind would weld on the hull with it sealed up and a fuel tank over it.

One teaspoon of fuel is enough

http://www2.worksafebc.com/PDFs/investigations/IIR2010154850011.pdf

Ive seen it with my own eyes on a boat here about 20 years ago, the photo still hangs on the wall of DOT Gero.

 I witnessed something similar years ago, my foreman welding up a leak in the sump of his mate's V8 Landcruiser. The sump was emptied of oil but they neglected to remove the oil filler to let the gases escape. Well the explosion was heard down the street. I witnessed it first hand, both the rocker covers were blown off. Lucky for him the sump didn't go the other way. One aspect was quite funny as he rolled out from under the car looking like a black and white minstrel, but a very lucky man indeed. I won't go near anything that's had oil or fuel in it, only if it's been professionaly degassed.

Old mate down at Augusta was altering a diesel fuel tank, welded and cut it on the Monday arvo, knocked off and started again on the Tuesday morning. More welding, knocked off for smoko then sparked an arc and it blew up, he lost half the fingers on both hands, an ear, lots of broken ribs and cuts and bruises to keep him in hospital for 3 weeks, lucky to be alive. Everything was ok till the fuel/ air mixture got to the explosive level then bang, off it went.

Had a Penguin ali that was at least 20 years old at time with in built tank that had the same type of pitted hole as in your picture. Initially had it welded on outside only by a "backyard" welder and that was ok for a few years but eventually corroded again. Then had it done professionally by a guy in Rockingham. He cut out an inspection plate in the floor above the hole as was  concerned about how far the corrosion went and warned me he could be "opening a can of worms". On opening fortunately all was excellent apart from that hole, welded it  inside and outside and all still good when I sold boat about a year ago. Can remember he was very safety conscious about how he went about doing the job.  Hopefully your corrosion is also confined to that small area where the hole is.

Going by the picture , that is a lot of pitting on the hull and seems like you have a serious problem .

I see this type of corrosion alot but ussually on the inside of the hull not on the outside. I am wondering if you have a earth wire to the hull ? If you earth to the alloy this will speed up the corrosion and create pitting all over.

Also by putting silicon over the holes you have contaminated the alloy even more, this can make the repair even harder to deal with.

As others have said, you realy should have a pro look at the hull to work out the best way to repair it, and find the cause of the problem to stop future corrosion.

 hey mate sorry just saw this post. That seems to be a bit of a problem seems like you definatly have an earth leakage which will eat through your hull quicker then my ex at an all you can eat buffet. since I can see you have already covered it in sika the quickest way just to do a quick fix would get a sheet of ali and arildite the f**k out of it but this is only gonna last a very short time you must find the earth leakage or there wont be a boat left soon. but to fix it properly u will have to pull up the deck it is alot more simple then it sounds if you want it fixed properly and very quick come in and see us. You prob wont even miss a day out on the ocean. Any more questions mate just message me :)