Ebay has the CP180 with external antenna for $399 right now. Depending on whether or not something would be overhead such a as hardtop or bimini an external antenna may be a better choice.
http://www.ebay.com/itm/Standard-Hor...item35bc3cd6ad
I have the CP180I in one of my boats and I love the functionality, however sometimes I have blips in signal.
I have had the CP155C and CP175C in the past as well.