Make a taming pen that is at least 6 fences high so they can't jump out then go around the swamp with an argy. Once you find one Lure it out of the water then pick it up and throw it into the pen. And since you can shoot crossbow bolts through the fence it should be easy to knock out. Just don't let it pick you up in its mouth
Here’s how I did it.
Step 1. Find low lvl Kapro
Step 2. Run at Kapro
Step 3. Punch Kapro til it faints
Step 4. Shove boa meat down it’s throat
This method worked twice for me, with a lvl 1 and a lvl 3. If you’re going for a high lvl, a crap load a health will see that this method still works
If your on the island go to the east side of the map. Go to the edge of the swamp and make a trap, even a two by two thatch one will work. Put door frames on the edges of the foundations, then finally a ramp for it to walk up on. You can also use this as a safety area to knock it out from. Vote ^ so others can see!!