I have the same issue & got some interesting suggestions from this forum a few months ago. As usual you ask 5 people & get 5 different views so here goes with what I decided.
My marine engineer mate with no vested interest in what I do said it was good practice to flush the ob through each time with fresh water or expect c 1/3 of its useful life - ie, it will last 3 times as long if flushed through each time. The odd day ok, but try & get it flushed through if you want to keep it for the long term (I do).
Anyway, looked at following options in abscence of tap:
25l container I got from a skip, washed it out & store full of fresh water when I fill up with fuel on way to slip. Any bigger is a bit heavy.
Above gives c1-2 mins flushing time, either do it manually or get a small submersible pump.
Manual - I bought a weedkiller container (16l I think) with pump, the sort you put on your back with a lance to spray the weeds. This cost 40e from garden centre & I took lance off & fitted a hose & side flush adaptor. I pump through the water by hand, get a few funny looks as apparently I look like a cross between Darth Vader & Ghostbusters. The pump action I do sitting on the sponson & the flow rate from the tell tale is good, ie, more than trickle. As the water has been stored in my car it is warm-hot (45 deg c) so should be more effective.
Auto - buy a Whale submersible pump & hook up to battery. Use same hose & side adaptor.
Some times I just cannot be bothered as it has been a long hot day but it is 3 mins at most. I reckon the salt on the boat & seats can wait but the damage is already being done inside the ob.
Hope it helps, can post a pic as we are out saturday.