Looks pretty straight forward, Here's a link for the new sending unit, assuming you have checked all the connections behind the panel at the water gauge? Otherwise, it looks like you simply cut the old one out, clean up the surface area, and mount the new one, I would guess with a strong silicone or 4200. https://www.fisheriessupply.com/kus-americas-s5u-series-npt-threaded-fuel-water-tank-sensor/s5u8
For the price, it's and easy decision to eliminate the sending unit, which if I was to lay money on it, that's the issue. Good luck!
