I am a relatively new diver (30 dives) and I would recommend Dive Palancar at the Occidental. I don't know who the dive op is at Iberostar so can't comment on them. DP should be able to pick you up from Iberostar as they are relatively close.
I consider myself a high maintenance diver. I change weight every year gaining or losing between 10-20lbs which means different wetsuits, different weights, slow descents, sometimes running out of air early, etc. Through it all the DMs at Dive Palancar were great. In fact at one point I had my camera (cheapo one that I bought at the dive shop) float up to the surface. I thought it was gone but the DM went up and 5 mins later came back with it. Not a minute later a turtle swam up next to me and he clicked a shot. It is one of the most valuable pictures I have today. That's what I call service. The people at the shop and the boat crew were great as well. They helped carry my gear on and off the boat.
I am going back in April and staying at the Fiesta Americana but will use DP for my dives. It is considered good practice to tip the DM (who shares it with the crew) about $5 per dive. DP dives are $32 per dive or $300 for 10 dives I believe.
As far as sea sickness, it is definitely not in my head. I get very sick on boats and once threw up under water through the regulator (not pretty). Anyway, that was before I discovered non-drowsy dramamine. I will check out the patch mentioned in this thread. I take the non-drowsy dramamine 1 hour before leaving and I'm good for the two-tank dive. DP returns to the beach for the surface interval so you can wait on shore instead of on the boat.
The water temps are in the mid-high 70s in March-April. If you or your gf are non-standard sizes, you may want to bring wetsuits. I am a curvy size 14 and had a very hard time finding a rental suit. Now I have my own suit and it makes a huge difference in how warm I am during dives.
Bring a lock for the lockers if you use DP. You can store your gear in the lockers next to the dive shop so you don't have to lug it back and forth from your room.
Have a great trip!