Marbelite pool - losing water

[eddyr]

Hi all.
We inherited a marbelite pool when buying some land off a neighbour, had it probably 7/8 yrs now with no real dramas. I hazard the pool must be around 25 yrs old - its looking (visibly) in good nick, the filtration was all renewed a few years ago, so the only original component is the pool itself.
Last year, around mid season - we started seeing water loss. Immediate thing we had done was have all the pipes tested, they were all fine.
We then dropped the level a little and had a good look around for any cracking in the shell. There was a number of what looked largely superficial hairline cracks, they didn't even look like they went through very far. We wern't sure these were the cause, as they extended far below how far the water would drop to.

We decided to leave it for the season to run out (crap weather last summer anyway!) and relook at it in the spring. So, here we are, some advice if you please.
We had a guy round to look (last summer) who wasn't overly helpful but did suggest it could be any of those small innocuous looking cracks. I'm not convinced, but is perhaps worth a shot. What's the best item for filling this? I guess you just gauge out a track wider than the crack itself and fill with a filler of sorts.

Another option i've considered is the blue rubber paint. Basically, due to the age of the pool - the marbelite has worn down a lot, so you've got lots of the hard fragments showing slightly pronounced where the fine plaster its mixed in has worn down, its like sandpaper. It can make it very hard to clean as algae etc gets in nicely, and its not very pleasant if you do happen to scrape along the wall/floor for whatever reason. That said, its very grippy!
My idea with the rubberised pool paint is two-fold, a) to seal the pool all over, as I'm guessing its getting porous with age? (I don't know) and b) to give the pool a new surface thats smooth, easy to clean.
Part of me thinks the paint would be too easy to solve the water drop problem.

So any suggestions/advice welcome! The alternative obviously is to have the whole thing restored, I'm not sure what that would involve but I'm guessing a lot of work and a lot of expense - something that I'm sure everyone can appreciate, avoided if possible!
Many Thanks