You could try one of these. As long as your current draw is less than 30A. It will also allow you to run 3s later on if you want.

[link]http://www.hobbyking.com/hobbycity/store/uh_viewItem.asp?idProduct=6468&Product_Name=Turnig y_30A_BRUSHED_ESC[/link]

It has a 3.0 V/cell cut out. but you should bring the boat in before you hit the cutout.

You could also use this as an alarm on your Proboat ESC. I have one for use in my scale models but it may be hard to hear in a sealed up fast boat. Works for 2S 3S 4S packs. It also starts alarming at 3.3V/cell.

[link]http://www.himodel.com/electric/Low_Voltage_Buzzer_for_2S_3S_4S_Lipo_Battery.html[/link]

Cheers.
Paul.