The compass always points to Terrapin.
The rulebook clearly states in both parenthetical text and body text that you can only buy one hacienda or water tile per turn. The rules concerning harvesting only have the parenthetical text but otherwise make no mention of having a limitation.
Where's the confusion? Its description in the rules very clearly says "once per turn" (emphasized as in the rules). That it's a "weak" choice is irrelevant.
That said, play it however you'd like.