Safety aside, 220v is more efficient.
Believe it or not if you are talking about power outside of the pond then 220v is actually safer then 110v. But in water if concerned about safety use 110v with a GFCI connection as the presence of water negates the saftey aspects of 220v wiring. (Lethality of either is based on duration of contact for the most part. 110v is harder to let go of when shorted then 220v due to the difference is muscle reaction)
If you are going to use a fountain water pump and use the pond for swimming then personally I would do as big_pond mentions and use 110v with a GCFI wired in the circuit. (I've seen unprotected 110v short in water and I have to say I wouldn't want to be anywhere near it). If you are using an aeration pump not in water I would go 220v if possible.
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