After you're done chilling in the cove, go for a 20 minute cruise (or do another set) to let the engine recharge the batteries.
We have a few miles to come in and usually ride all the way in so 30-45min. For some reason they just never seem to hold up long. I might get half an hour of water time in and then done. Its not a monster system at all. Maybe I'm just expecting too much and need to stick a cheap boombox in the boat for such occasions.