A few options:
National trust fell foot park - info on NT fell foot park website it’s called ‘active base’ if you’re doing a on line search.
Low wood water sports centre.
Lake wardens do some storage for small craft at ferry nab - prices/ info on LDNP website.
Low Wray campsite - charge a weekly rate for taking a boat whilst you’re staying there but who knows, you may be able to negotiate a storage fee.