score:6
The monsoon rains generally do not start until the end of May in that part of the world. What this means for you is that, the ocean on the western side of Sri Lanka is generally calm until June. So, I would say any beaches in and around Colombo or below would be great to check out.
As a bonus, you might be able to do some "blue whale" watching, if you go to the southern side of Sri Lanka (Trip advisor Link).