Soapy water sponged on to the inflated boat. Don't forget close attention to the valves where the leak most probably lies. New valves can be easily applied for most makes.
Silicone grease round the threads and flap on the valve works wonders. One of the best sources for silicone grease is a plumbers merchants who stock it for use on lubricating underground drainage pipes prior to pushing together
For what its worth!
I heard or read somewhere that 'brake fluid' was good on older inflatable valves, Something to do with its rubber/plastic preservering property?.