One thing to remember when going for a swim is that there are many sharp corals near the beach, especially near Tongatapu and PangaiMotu. It is a good idea to wear a cheap pair of sandals while in the water. There are jelly fish and they do sting! They are also hard to see. It is a good idea to have a bottle of vinegar handy in your bag to help treat any stings.
There is no malaria in Tonga.
Exercise the usual caution when snorkelling as the coral can be dangerous.
respect
For maximum respect, keep your knees covered both men and women. Men, keep your shirt on everywhere except at the beach. Topless men off cruise liners have been arrested and held until after the ship has left! This is a very conservative Christian country. Keep in mind that Sunday is strongly revered, the vast majority of the population will attend religious services, very few shops will be open and there is very little to do. Hotels will be open, as will some restaurant and beach resorts, although mainly to serve expats and tourists. Small shops, including, in Nuku'alofa, a popular bakery, may open later on Sunday afternoon.
TV stations close or play Christian shows on Sundays. Radio stations will also play religious on Sundays. To compensate, the cinema in Nuku'alofa usually has a screening just after midnight on Monday morning.
Tonga features many major Christian denominations; the Latter-Day Saint church especially has a widespread presence. Many of the services are very enjoyable. Strike up a friendship with some locals and you will have no problems finding an enjoyable Sunday experience despite the lack of commercial activity.
talk
Tongan is the most widely spoken language in Tonga. English is also widely understood because many of the high schools teach exclusively in English. Many Tongans when asked a question they are unsure of or don`t understand will reply with a "Yes". In this case, ask a follow up question and if the reply is still "Yes", ask someone else.