These may take on some water if left floating. But, if your battery is in good shape, you should be fine for 3 days. IF the water gets deep enough to trigger the bilge pumps, it should suck the water out in 30 seconds or so and get it below the bilge trigger depth.
I wouldn't leave it in the water for, let's say, a month, without running it every other day or so. Once that battery dies, it will keep filling up and potentially sink.
Pretty cool. Huh? A boat that WANTS to sink...:ack:
But not all seals leak at rest. So you may want to at least check on it after a few hours and see if yours is leaking.
My experience with this was luckily on dry land. I had the boat trailered and in my driveway, nose up, and cover on. Bilge plugs open. Came out next day after a rain storm and battery was dead. Experimented and found that the rain had filled the engine bay enough to trigger the bilge pump, but the trigger was set so that even when the water was below the trigger, it would not turn off the bilge. So, dead battery, dead pump. Had to put in a new pump and bend the trigger so that it would go off about 15 seconds after it quit sucking water.
Like I said, lucky to find this stuff out on dry land, and not sitting in water.