I am planning on spending some time in Playa soon and this is what I thought about doing to get some diving in. Arrange a 2 tank twilight/night dive one evening and a two tank morning dive the following morning. Stay in an inexpensive hotel as mentioned before.
If you must stay primarily in Playa, that sounds like a good plan to maximize you time in Cozumel. Essentially two days of diving for one ferry crossing. The savings from a ferry crossing will halfway cover a cheap room somewhere like Suites Bahia.