Specs in a 1996 boat test say 160L, however things can change in 4 yrs - up or down! If in doubt, drain it out & fill from empty.
OR, if you can easily get to the tank to measure it! Measure H, W & D in mm, take 8mm off all three dimensions (assuming 4mm thick), convert any two of them to 'meters' and multiply m x m x mm and you will get the internal tank volume in litres. This won't allow for the baffles, but it will be close enough.
cheers