haven't tried sabang divers but otherwise diving is PG will be pretty great...
i think ATMs are a problem on PG... so best bring a good amount of cash to cover expenses, but i think there are several money changers there who take travelers cheques
other things would be specialized or prescription medications your guests would need - the common stuff (cough, cold meds, upset stomach, multivitamins, rubbers, feminine hygiene things are easy to get... not tampons though, even in manila they're hard to find... it's a napkin country you see... so if you have women who are expecting during your trip best to bring a supply)
hope this helps
Jag