About 27 km away from Jaffna town, Casuarina Beach takes roughly 45 minutes to reach via local bus. Luckily if you're in town, no bus changes are required and you can take the direct Karainagar bus which is usually not too crowded at the start of the journey.
The beach is situated on the Karainagar island, which is accessible from the mainland via the Ponnalai Causeway (AB17). The drive itself is lovely, with miles of saltpans and water on either side. About 10 square miles of paddy fields and farmland, the island's only real tourist draw is the Casuarina Beach and the little Fort Hammenhiel Resort & Restaurant, situated just off the South coast of the island. Unfortunately we couldn't make it to Hammenhiel but it's supposed to be a lovely lunch spot after a morning of beaching.
Casuarina is one of the few places for locals to let it all hang out in Jaffna – although not too much. Alcohol is strictly prohibited on the beach, and most people in the water appeared to be men. One or two fully-clothed women wafted near the shore, but the majority waited on land with towels and water for their frolicking menfolk. They do have "changing rooms", essentially a set of walls within which people get out of their wet clothes. Don't expect privacy or hygiene though. We went on a late Sunday morning, which is possibly one of the most crowded times. I assume early mornings and weekdays should be better though.
In ordinary circumstances, I would have cartwheeled into the pure blue water with glee because the heat was truly scorching, but we were getting enough stares as it was. Instead, we struck up conversation with one of the soft drink kade shop guys (the Blue Casuarina Restaurant – not a restaurant), a former Navy man, and plotted an alternate beach expedition. We ended up paying a fisherman about Rs. 1500 to take us a few kilometres into the beautiful azure waters so we could have a secluded swim.
It was a totally surreal experience- as we swam, we could see the shore some 3 to 4km away, but it was pretty shallow as we were on a reef of sorts. The water was bliss and the view was stunning. The feeling of being smack in the middle of the Indian Ocean, with nothing but a few kilometers of water between me and the coasts of two countries was strange to say the least.
TL;DR – This isn't a bikinis and beers sort of beach. Go early in the morning, or bribe a fisherman to boat you to the middle of the ocean for a swim.