The reality is that damage to hire boats is either a broken cup or scratched bow OR a flooding/rudder damage. There isn't a lot in between other than the popular blocked toilet routine. So - if you are confident, negotiate, offer a deposit in lieu of damage waiver. If not pay the £50 and don't worry for the rest of your holiday (unless of course the small print has exceptions, in which case ask exactly what damage you are waivering)
Chasing guests for damage incurred is very difficult and some hire companies will treat the combined waivers over the year as the winter maintenance pot.