I use a #330 body grip and set them in travel spots. You can set this way at location along the bank where they are leaving/entering the water or in a channel they are using. For along the bank I set the trap a few feet from the shore completely submerged. You should make a funnel with sticks to prevent the beaver from swimming around the trap. Also set a stick across the top of the trap (dont interfere with the dog) so thet the beaver will dive and swim though the trap. In a channel just set and secure the trap in the channel. Alsways tie the trap off to something to a tree or secured anchor spot.
Check you state regulations about setting on a dam and if you can remove sticks. We cant do this in NY. If legal, you can remove a section of the dam to let the water flow out and place a #330 in the whole you just created. The beavers will want to plug that up ASAP.