You can get a float switch MO for pretty cheap, or just make your own. Check the DIY section. They also have electric topoffs, but I don't know how much they cost. I have always played around with the notion of using a float switch for a sump pump, but don't know how corrosion resistant they are to salt water.
Many people use the Kent float valve. No pump and no electricity involved. This subject has been discussed on this board several times so you may want to do a search.