Mostly. Ferries to a number of Thai islands have sunk in the past and overloading remains a problem, particularly with longtails. However these boats do not sink on a regular basis and you'd need to be escepially unlucky to be on a Thai ferry that did sink.
A lot of it comes down to common sense -- if the boat is overloaded, the boat pilot visibly drunk, most of the passengers drunk or the weather very bad, then don't get on the boat. Catching a speedboat late at night from Ko Pha Ngan's Full Moon Party to get back to Samui is not recommended -- these boats are dangerous and the passengers often not fully in control of themselves.
Travel by boat is generally quite inexpensive and the traffic frequent enough that at worst, you'll only be waiting till the next day for another departure.
Routes that we know boats have sunk on are Phuket to Ko Phi Phi, Phuket to Ko Yao Noi, Ko Tao to Chumphon and Ko Pha Ngan to Ko Samui -- though there are probably others as well.
Please find below some of the more frequently asked questions (FAQ) people have about travelling to Thailand. We've tried to answer all the most frequently asked questions, but if you have another query about Thailand, please try our Travelfish messageboard.
Is it possible to rent rooms or dormitories for long periods of time?
Rooms: is a fan room ok, or should I budget for air-con?
Should I take my own padlock for the door of my room while travelling in Thailand?
What should I do if a bag is stolen from my room or dormitory while travelling in Thailand?
What standard of acommodation can I expect for US$5?
Why are there so few dormitories in Thailand?
Can I drink the tap water in Thailand?
How can I avoid MSG in Thailand?
How do I avoid peanuts in Thailand?
I have a food allergy - will I cope?
Is there good vegetarian food in Thailand?
Is western food available in Thailand?
Are the trains in Thailand safe?
Do they trains run on time?
Is it easy to hitch-hike in Thailand?
Is it easy to rent a car or motorcycle in Thailand?
Should I get a tourist minibus in Thailand?
What is Bangkok's bus system like?
Which is the better way to travel around Thailand - train or bus?
Are credit cards accepted in Thailand?
Are there ATMs in Thailand?
How can I have money sent to me in Thailand?
How can I send money out of Thailand?
Should I bring travellers cheques, credit/debit cards or cash to Thailand?
Should I tip in Thailand?
Are leeches a problem in Thailand?
Are legal drugs cheap in Thailand?
Do I need a prescription in Thailand?
Do I need malarial medication in Thailand?
General food and water precautions in Thailand
General health advice for Thailand
HIV/AIDS in Thailand
Is cholera a problem?
Is heatstroke a problem in Thailand?
Is medical care cheap in Thailand?
Is medical care in Thailand good?
Is rabies a problem in Thailand?
Schistosomiasis in Thailand?
Traveller's medical kit for travel in Thailand
What about dengue fever in Thailand?
Will I get diarrhoea in Thailand?
Are there many internet cafes in Thailand?
Can I travel with a laptop in Thailand?
Is there ADSL internet access in Thailand?
What is mobile phone coverage like in Thailand?
What is the voltage in Thailand?