Snorkeling and diving with whales is currently prohibited in Sri Lanka. Special expedition permits are sometimes issued for small, high end groups and film crews. There are many local operators running snorkeling trips but these are technically not licensed and are illegal.
While my personal opinion is that a well regulated whale snorkeling experience should be allowed the fact is it is currently not and as such I would not endorse an unlicensed activity. In Mirissa it is also complicated as there are many whale watching boats, and I would be hesitant to be in the water with those boats around. The whale watching is poorly regulated and while there are a few good operators, most are reckless, harass the whales, and have poor safety standards or whale watching protocols other than chasing after the whales aggressively.
If you are interested in general diving Mirissa has a few good spots but there are better places. Hikkaduwa and Bentota for some nice underwater rock formations, the east coast for coral reefs, and Colombo for wrecks.