What is the difference between an inground and above ground pool heater?

If you are looking for a gas heater, as long as it is listed to US Standards, and identified as a “Pool Heater”, one should be as safe as the other. Just be sure you cross-check the Manufacturer’s Instructions to verify it may be used for either installation.

If it is an electric heater, the following is taken right out of the UL WhiteBook - “Guide Information for Electrical Equipment”. If it meets these Standards, it is safe when installed correctly.

“HEATERS (WBRR)
This listing covers heaters intended for permanent installation in or adjacent
to swimming pools or spas. Heaters for hydromassage bathtubs are
covered as part of the hydromassage bathtub Listing.
Products listed under this category have not been evaluated for outdoor
use, unless they are marked ‘‘For Outdoor Use’’ or equivalent, in which case
they have been found acceptable for both outdoor and indoor use.
For additional information, see Electrical Equipment for Use in Ordinary
Locations (AALZ) and Plumbing and Associated Products (AAPP).
The basic standard used to investigate products in this category is UL
1261, ‘‘Electric Water Heaters for Pools and Tubs’’.
The Listing Mark of Underwriters Laboratories Inc. on the product is the
only method provided by UL to identify products manufactured under its

Listing and Follow-Up Service. The Listing Mark for these products
includes the UL symbol (as illustrated in the Introduction of this Directory)
together with the word “LISTED”, a control number, and the product
name ‘‘Swimming Pool Heater’’, ‘‘Spa Heater’’ .”

This means it will be safe when installed according to the Manufacturer’s Instructions, per the Listing, and according to the requirements of NEC Article 680.

How efficient it is, I can’t tell you.

200,000 BTU sounds like a lot for a pool heater (I usually see them around 100,000 BTU); water temperature stays pretty stable, compared to the ambient temperature of the surrounding air. Have you considered a solar cover and a smaller heater? The covers are generally installed with in-ground pools, but are available for above-ground use, also.

Whether it is electric or gas-fired, it must be bonded to the pool’s bonding grid. Some gas models come with a straight 120 Volt attachment plug on a cord. Any cord connected equipment, other than the pump motor must be plugged into a GFCI protected receptacle at least 10′ away from the pool (and the metallic box must be picked up by the equipment grounding conductor for the circuit). Most heaters are hardwired, whether electric or gas. Simply removing the cord and plug attachment and hardwiring the heater may void its listing, rendering it unsafe (or, without a means of proving it is safe, since it was evaluated only for cord and plug attachment). Unless both the pump manufacturer and the heater manufacturer state that the heater is allowed to be on the same circuit as the pump, it must be on its own circuit.