Thanks everyone for ideas
I've decided I'm going to try hot glue, but there's another thing I forgot to add in my earlier post, there's a small hole in the bow of the boat, just above the waterline - it's there by design and someone said it's probably a breather hole for when the battery is charging, but considering the battery is completely sealed, my guess is that it's a drain hole for when small amounts of water get inside, and indeed it does - mostly through this effin hole, so my next question is will hot glue retain it's strength when immersed in water?
The water can't do any damage inside the boat as the pumps and other electric shenanigans are raised up about 90mm from the floor, but obvioulsy the battery is sitting directly on the floor.
Even if I don't glue the battery directly down, but decide to go the route of fitting a plastic 90 deg angle to each side of it with a strap, these will need to be glued down, and if I do go down this road, I'm not too bothered whether the glue sets like cast iron as the battery will be easily replacabubble.
I saw some gorrilla glue yesterday, it was a fiver, is this about right?