I was to go and do an internship in Cancun which did not pan out for me, after traveling a bit around mexico and DR, I came back to finish my course, Neptunes had been very nice and helpful and decided to stay with them.
The dive center is nothing like your typical dive shop I have ever seen it is definately more like a school, very little retail, the hotel is amazing and the trips are well organized, they better be as the interns are responsible for the day trips. BTW I thought it would be free labor, but once you get dive master your given compensation for the work, which is the same amount as the staff so its nice.
Manzanillo is not a crazy place so if your looking for the wild side or the decadent crazed sexual/tourism adventures better go to thailand (next stop for me). There is a ton to do after your training and as the staff is always making trips to other places, the interns go so it always becomes like a field trip, I have learned so much on these trips and So your never bored.
The downside to this place is there could be more nightlife but hell we are here to become dive professionals, the people are really nice, all the guys have girlfriends within a week of being here. There is evidently something appealing to the local ladies if your a foreigner. Well, the truth is I recommend this place, the training is amazing a bit overwhelming at times but I think worth it.
PM if you have any questions. good luck with your search.