"Poon" is definitely derived from "poontang", which is used only in North America, AFAIK. Coomon synonyms of "poontang" are "nookie" and "tail".
Not only that, "poon" as a verb and "pooner" as a noun seem to be used only on PERB. You won't even find those words on TERB and MERB. Elsewhere in North America, on other boards, the most common equivalents are "hobby" and "hobbyist". In the UK, the equivalent word to "pooner" is "punter".
On the World Sex Guide, the equivalents are usually "monger" as both a verb and noun. (it comes from the Biblical term "whoremonger").