Start with the world, because it explains why April is worth considering here at all. Of the seventeen coasts with a season recorded, exactly one is at its own peak in April, and it is Ireland, at ten degrees. April is the gap in the global calendar: the northern winter has finished producing and the southern winter has not started. Nowhere is at its best, so the question stops being where is it good and becomes where is it least bad and warmest.
On that question Sri Lanka answers well. The north-east monsoon is letting go, so the offshore mornings on the south coast grow patchy through the month, and the south-west monsoon that will run the east coast has not arrived. But the south still works, especially in the first fortnight and early in the day, and it works with almost nobody on it. By the last week you check the wind before getting up rather than assuming it, and the reefs at Midigama hold shape longer than the open beach because they need less help.
The water reaches 30°C, the highest reading of the Sri Lankan year and warmer than most heated pools. Put that beside Ireland at ten and you have the shape of April: the one coast at its peak is one you surf in five millimetres of neoprene with a hood, and the one that is merely good is one you surf in a rash vest.
Book the south coast, book it early in the month, and treat Hiriketiya as the insurance. Two headlands wrap the swell into the cove, so it keeps a rideable wave in a week when the open coast has gone to pieces. Arugam Bay is six to seven hours across the interior and six weeks from starting, which makes it the wrong answer in April however good it looks in July.

