Hi,
Have just done the exact same thing at the weekend, with ok results, although not up in the air too long.
The zenmuse did handle it very well. One idea I had for the next time, was to possible set up on shore and then launch from the boat, that way I would have a RTH point if anything happened, but that would only work on an inshore situation, not right out at sea, and if you had someone at base to radio / phone if there was a problem to receive the phantom and keep the landing point safe etc. another idea was to take a board to place the phantom on and hold it level whilst it got the gps lock. - Both ideas untried though.