This is a question we hear often from people planning a Sri Lanka trip who want to dive. Both regions are good options. The right choice depends almost entirely on when you are visiting and what you want from the experience.
Trincomalee: the case for. HMS Hermes is the reason Trincomalee sits in a different category from most Sri Lankan dive destinations. A 182-metre aircraft carrier wreck at 35 metres, largely intact, covered in coral after 80 years underwater. Nothing else in Sri Lanka competes with this. Add Pigeon Island's reef sharks and hawksbill turtles, the blue whale watching in March and April, and the overall quality of a less-developed dive destination, and Trincomalee is the choice for serious divers or anyone who wants a centrepiece dive experience.
Trincomalee: the case against. The diving season is strict, May to October only. If your trip falls outside that window, it is not an option. Trincomalee is also a smaller, less tourist-developed town than the south coast. Accommodation options are narrower. Transport is more involved. If you are on a short Sri Lanka cultural tour, Trincomalee is geographically isolated from the main tourist circuit.
Hikkaduwa: the case for. The south coast is accessible for diving from November to April, is two hours from Colombo by train, and sits on a well-developed tourist strip with easy infrastructure. Hikkaduwa reef is a good beginner dive: shallow, calm, turtles everywhere. The SS Conch wreck at 30 metres is interesting and manageable. If you are doing a short Sri Lanka trip through Galle, Mirissa, and the south coast temples, Hikkaduwa diving makes sense.
Hikkaduwa: the case against. The best sites are busy. In peak tourist season (December to January), dive sites are noticeably crowded. Visibility averages 10-15 metres, good but not the 20-30 metres Trincomalee delivers in peak season. There is no equivalent to HMS Hermes. The dive infrastructure is more commercialised, with higher diver-to-instructor ratios and faster turnarounds.
Our verdict. If your travel window is May to October and diving quality matters to you, Trincomalee is the better choice, and not by a small margin. If your trip falls in November to April or you are already on the south coast, Hikkaduwa is good and worth doing. Some travellers manage both by doing the south coast first (October to April), then returning to Trincomalee from May onward.