avon searider holes blocking up ?

[avon5.4]

just had my 5.4 on river but noticed that the previous owner had drilled a drain hole into the floor of the rib inside a fuel tank box fiberglassed to transom, its ok when going but as soon as the hull fills up so does the box with the fuel tank in it. we have sealed the hole up with a stainless steel bolt and washers and plenty of silicone ,in a hope of keeping water out. is it possible to block up the front and back holes so it does ont flood when not needed ,but can remove if want to flood for when diving.

[rugrat]

i think there have been lots of threads on this subject already
try searching for "searider flooding hull"and you should get some results