For most small ponds, a submersible pump is sufficient, a prefilter can be attached to the pump to prevent the pump from fouling. With very large ponds, and external dry pump in a separate housing is the best way to go for easy maintenance. There are dozens of brands on the market. Many are oil filled, which can cause problems in the pond if they should leak. Epoxy encapsulated pumps are safer for aquatic life as they won't leak.
Some pumps are more energy efficiant than others.
An 800-gallon pond should have at least an 800 GPH pump. It is best to oversize the pump rather than undersize it. When using a biological filter, the flow through must be at a certain rate, so extreme oversizing is not recommended, or a separate pumping system may be desired.