Here's my standard answer:
Yes, you can get here by water taxi. We generally suggest that guests take the plane to get here and the water taxi on the way back. Here's why: when you arrive at Belize International (which is in Ladyville), you must go through immigration before you get your bags and then customs. If you take the water taxi, once you find your bags, you must take them outside, hail a cab and have the cab take you to the Water Taxi terminal in Belize City. The taxi ride is $25.00 US. There you get your bags, again, and take them to the pier where they are loaded onto the water taxi. The trip lasts about one hour and forty minutes. The water taxi can be very crowded. The boats are generally open without tarps. Once here, you have to get your bags again, and take them to another taxi. The taxi then takes you to your hotel. You can walk to your hotel, but even the best luggage doesn’t roll well on sand. The cost of the taxi to the water taxi terminal in Belize City is at least $25 US. The water taxi fare to the island is $30.00 per person. The taxi in San Pedro can run anywhere from $4 US to $20 US depending on where you are staying. On top of that, you have been traveling all day and you have no control over the weather. A two hour boat ride in 75 degree weather, if overcast, feels very wet and cold. Depending on your arrival time into Belize International, you may not make the last water taxi, which leaves at 4:30. The last plane out of International for San Pedro leaves at 5:30, and you arrive in San Pedro at 5:55.
If you take the taxi when you leave, the costs are the same, but at least you are "fresh", i.e., you are doing the above in reverse, but haven't traveled all day, and you will almost always have the option of taking the plane if the weather is not excellent.
The plane trip over is $65.00 US, You will only have to retrieve your luggage at the International airport, find a porter who will load your luggage onto a plane for San Pedro. Once here, another porter will retrieve your luggage from the airstrip and a taxi driver will load onto your taxi for your hotel.