I think La Paz does not have enough characters for the search engine. Search "cantamar" and you get some threads with good information.
I have only dived La Paz in September and October with Club Cantamar (
http://www.clubcantamar.com/english/index.html ). It looks like the water temperature in May is a little chilly (
http://www.clubcantamar.com/english/infos/index.html ) The resort is okay if you like "dive camp". It is functional and convenient for a dive vacation, but lacks the frills of a higher end resort. The dive operations were wonderful, and the diving was even better. The diving is good, damn good. Club Cantamar also offers a whale shark snorkel. They claim a 98% success rate on finding whale sharks. My experience has been 100%. If you decide on Club Cantamar, I would suggest you do the half board option, rent a car ($125-$150 a week), and go to downtown La Paz for dinner. There are a lot of nice restaurants in downtown La Paz. Club Cantamar is about 20 minutes North of La Paz. Cabs to and from Club Cantamar can be had for $20 each way. Club Cantamar offers two shuttle rides to La Paz per guest per week if you stay there.
If you like higher end resorts, then stay in La Paz. I have heard good things about La Concha (
http://www.laconcha.com/ ). There are also several dive operators you can use out of downtown La Paz, and Cortez Club comes to mind (
http://www.cortezclub.com/ ). They also have links to other hotels/resorts in La Paz on their website.
You can also stay in La Paz, and dive through Club Cantamar. They will pick you up, and take you back to your hotel if you dive through them.
I hope this will get you started. Feel free to post or PM any other questions, and I'll be happy to answer them.