white fresh water tanks are almost always polyethelyne (sp?) Good news is that it is easy to spin weld, bad news is that not many things stick to it long term. IMHO the best choice is to spin weld a patch onto it if possible. next would be to use a heat source to melt the plastic over the hole, same process as spin welding. Spin welding uses friction to melt the plastic and form a permanent bond.
Third choice would be to try JB weld or other things people have had good luck with.
Another option would be to drill out the hole to the size of the plug used for tank sensors and install a tank sensor sender. course that only works if it is on the side or top of the tank, not the bottom.