Just came back from Anse Marcel beach. Overall, we liked it. The biggest problem was seaweed. Thankfully it was mainly on Secrets resort side, which was almost unbearable - smell and to walk. On the Anse Marcel side it wasn't that bad, but it wasn't perfect either. Anyway, we decided to stay and got two chairs and umbrella for $37. Beach wasn't crowded so it was a plus. Water was murky so it was a minus, we couldn't snorkeling. Restaurant was a big hit! Lunch was excellent so as the service!
We made a reservation for tomorrow to go there again - and apparently you need reservation to get front row chairs, today we were on the second row, but it was perfectly fine - chairs were spaced out as it wasn't crowded.

I'm not sure if there are better alternatives close by ( took us 12 minutes drive from our hotel and no traffic). Pinel island? Not sure if we will go. We would still like clearer and calmer ocean, but at this point I don't think it exists on St. Marteen:-)
Overall, beach and especially restaurant fit our needs 80 % so we might go there for our last 3 days on the island.