I made this mistake by ignoring other people's warnings by convincing myself that it couldn't be "that bad". It is; in fact, it is probably worse.
The fisherman's cooperative in Taganga (one hour drive from Tayrona Park entrance, 15 minutes from downtown Santa Marta) takes passengers every day around 9-9:30 am to San Juan del Cabo in the Tayrona National Park for the price of 40,000 pesos per person THERE and 20,000 pesos for the ride back (this is probably because you couldn't PAY me to go back that way and as a result, have had to provide an incentive by reducing prices).
You must purchase a garbage bag in which you will put all of your belongings to "protect them from getting wet" which may not be very effective.
BE WARNED: The ride is VERY VERY VERY rough. Every day. The waters along this route are always rough (even on a nice, beautiful day like we had). Not only may you get seasick but we were physically hurt by being flung up and down into the boat like that. My friend got a concussion. The anchor almost hit us while we were flying up and down straight into the waves. ALSO, Colombian rules require all passengers to have lifejackets ON, that the boat have TWO functioning outboard motors, and limit capacity (to 12 people per boat including crew I believe). If any of these are violated, you may be stopped, fined and who knows what else by the Colombian Coast Guard. Our boat was over-capacity (we didn't know until we were told to lay down in the boat). We spent over an hour hiding in a cove from Coast Guard and then trying to outrun them.
IF YOU MUST: Don't pay up front despite their insistence, as you may not get there at all or in one piece. Sit as close to the back as the boat as possible as this reduces the roughness of the sea on your body, and DONT EXPECT NOT TO PAY THE PARK ENTRANCE FEE. The park officials are onto this game and will likely be waiting for you at the beach to collect the 31,000 pesos.
There are several areas in the Tayrona Park that are not for swimming due to the very unpredictable and strong currents. Particularly in Arrecifes, numerous lives have been taken here. As a result, it is best to swim only in La Piscina and San Juan and only during the day.