Water/refrigerant heat exchangers are significantly smaller than air coils due to more efficient heat transfer. Tube in tube exchangers are the least expensive for your application. A shell over tube exchanger might be needed for the sea water unit so you can clean it. Sea water heat exchangers are expensive.
The newer rotary compressors are somewhat more efficient than piston units. Yours may be rotary if the system isn't very old. As Mike said, the biggest gain in efficiency with the latest systems is primarily due to larger heat exchangers. This mainly applies to air coils where the difference in size can be significant.
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