Google search brings up a lot of options.. Cell boosters on yachts are common. You need to get something that is Marine/Yacht specific as you will be dealing with an omni directional antenna/receiver as well as it needs to be able to handle moisture/salt/corrosion.

Also.. No matter what system you use.. eventually if you get a certain distance out from land it just isn't going to work. Be sure to limit your clients expectations or this will become a recurring thorn in your side.

[Link: alternativewireless.com]