This answer depends on the method of installation and maintenance and the type of water you’ll be swimming in.
One of the biggest disadvantages of the twisting method is that you cannot swim. However, all other methods you are able to swim with your dreads.
The second concern is the type of water. Salt water pools and the ocean are by far the best bodies of water to swim in with dreadlocks. Ponds, lakes, and other natural still bodies of water are things you want to avoid due to living organisms in the water. If you swim in one of these it is recommended that you do a deep cleanse soon after. Swimming in chlorinated pools are OK, but not recommended.