Just returned from a 7 day trip to HMB. Also stayed there 13 years ago, so would like to share my experiences with fellow travelers. First, the grass is bad in many, many places along the YP. The intensity seems to vary - based on the beach and wind direction, available labor and just pure bad luck. A beachside breakfast (7/31) near the PDC ferry pier revealed 20 yards of brown water and a mild amount of sargasso. Akumal's sargasso issues were present each day, although the intensity varied. Please note that not only are you dealing with 8-12" deep matting in places, but the stench can be repugnant. Again, intensity depends on the above factors.
Now for HMB. Grass was covering the middle third of the bay. On some days this same section experienced matting (and brown water) stretching out 40-50 yards from the shore. The breadth would change day 2 day. Also dotting the beach were piles of rotting sargasso which caused the intense smell. The worst was centered around La Lunita. Of special note, the beach laborers were working heroically to remove the piles but were obviously fighting a losing battle. On a good note, the northern third of the bay (iron shore) had very little grass issue and the water was as beautiful as I remember 13 years ago. We stayed at La Sirena, and enjoyed our stay immensely. Cannot recommend this property enough. Great location and property. For us, the grass was merely a nuisance while snorkeling in HMB. It appears that the iron shore's shallow reef protects this section of HMB. From our vantage point, we watched wave after wave of grass float into the southern 2/3's of HMB. The snorkeling in HMB was a mixed bag. Made 8 trips and saw no turtles. 13 years ago I saw a half dozen in just 3 trips. It appears that most of grass they feed on was absent. That said, I only covered the northern 2/3's of the bay. We did see 2 nurse sharks, one a good 5-6 footer along with what looked a barracuda. Also saw many rays, including southern, yellow and a small eagle ray. General fish population was good. To summarize, we had a great trip. With some research, we successfully targeted La Sirena as our defense against the grass's ill effects. The people and food were great. Weather was perfect. I would not hesitate to go again in the future. I hope this helps those considering a visit to Half Moon Bay.