The bottom could start to delaminate at speed and eventually throw everyone from the boat , thats all.
Counter sink the holes and use sloted counter sunk screws .
Then use some Poxy Putty liberally over the top of the screw heads .
Sand the excess Poxy Putty smooth and you should be OK .

-beerjet-

I dont like the whole drilling through the bottom of the boat idea but .......
Someone used some metal insert type things once .
I think they glasses down a healthy piece of half inch plywood and these would sit under the glassed in wood with just the female end of it exposed .
Then you could use a good sized bolt to hold down the braces and everything is in the boat and not through the boat .
I'll see if I cant scare up a picture of what I'm talkin about .

-beerjet-

Sometimes you can find these with barbs on the washer end .
Cut your plywood to your desired size .
Drill a hole for your insert to fit into nice and snug . If you can find these with the barbed end , you can hammer them in and they are in there tight and no worries of them spinning .
Glass in the entire wood peice , preferably putting a 45 degree angle on all four sides for a smoother transition of all your edges and more surface area making contact.
Now you'll have somewhere to bolt your tanks down and nothing going through the hull .

Your best bet is to test fit the wood to make sure your tanks arent too far in any direction before you glass the plywood in .
Ideally , this is what I would do .

-beerjet-