Portable 16,000 Grain Mobile-soft-water(tm) Unit with Tank, Tank Head, Lead Free NSF 61 Male GHC Tank Connections, Distributor, Resin and Instructions. Used in Rv & Car Wash Applications.
I bought one of these from Amazon for $215 a few weeks ago and am very pleased with the results so far. The water has been very soft and it has not needed recharging yet.
Basically, it is recharged by adding a couple of boxes of common salt (#4 for this unit). You dump the salt in from the top and wait for 10 minutes. Then a trickle of water is added to mix the salt around the resin for 5 minutes and then you flush it a little bit faster until the water doesn not taste salty. I am not sure about recharging with the oceans saltwater. I do not think you would want to recharge this on a passage due to fresh water wasting but it should be good for the onboard tankage volume. at a 30 grain hardness it would soften about 500 gallons.