I love Sarteneja village, but I don't know of any dive shops there or of any nearby diving. For diving you'd have to go to Bacalar Chico park or elsewhere on the east side of Ambergris or beyond, not exactly close by. Blue Ventures, based I think in England, runs "volunteer" programs out of Sarteneja (but you have to pay to volunteer) and has a dive component, but so far as I know the dive part is or at least was based at Bacalar.