http://www.greenbuildingtalk.com/tabid/53/afv/topic/aff/21/aft/76625/Default.aspx

Can anyone provide feedback on the feasibility and typical cost of a Geothermal system, keeping in mind:
a) Im building on "filled" land (marl & other aggregates) right on the water (canal) so Im not sure if the thermal properties or depth requirement are doable.
b) Performance wise Im not sure how reliable it would be to cool a home in 80-90 degree temps year round (as compared to HVAC). Im in Carribbean
c) Im not familiar with what would be an optimal setup of a GT system in terms of materials, design, etc.

High efficiency air source may be a better bet in your circumstances, although if you are near ocean, salt air tends to quickly corrode outdoor unit aircoils, a significant advantage of geo.

Horizontal systems here in Florida are said to need 1000' per ton - I've never done one. Wet soil is helpful for heat transfer, may hinder installation. Your deep ground temp may be 75-80 F, another important design consideration.

Some air source brands may be better at resisting salt air than others. Local experience might me useful.