I use wire rather than cable, can cut to whatever length you need on site. But you can use cable with this if you prefer. Sounds like you have dirt or mud bottoms if there are no rocks on site. Just cut a long stake for the deep and attach the wire/cable to the bottom of it before you drive it in. You can cut the stake whatever length necessary to have the top you are pounding on above water. And if you are using wire you can make the drowning wire whatever length necessary to reach deep water. If it is six feet deep, three foot off the bank, you can just cut an eight foot long stake and anchor it there, or if you have to go fifteen feet out to get to drowning depth water, just cut your wire that long. When you make a catch, pull the bank stake and using it as a handle give a few sharp yanks and it should loosen the drowning stake enough you can pull it out.

You can drown beaver in 16-18" of water if you make sure and front foot them, just make sure you are setting for front feet if using a drowning setup in shallower water.

I've never trapped beaver out of a boat, but I've done plenty of it on foot, and packing a roll of wire, a cheap pair of pliers/wire cutters (so you aren't out much when you drop them in the drink) and an axe sure beats packing a bunch of rods or weights, assume it would in a boat also.

I’m probably not much help… but I don’t use drowning rods nor premade cable drowners because the conditions sometimes aren’t right for them.
I make my drowners on site,, I use 14 gauge wire DOUBLED and use sand bags and pogos.
I fill my bags with sand or gravel using a mini shovel that I picked up for 11 bucks at harbor freight.
You can fill up a sack pretty quickly using it.
I don’t like to use a drowning system if the water isn’t at least belly button deep( I’m 5’11).
I use pogos do cut down on weight and they hold surprisingly well if you go pretty deep with them.
I wire the pogo stake and pound it down at least 18-20 . The 1 1/2 inches sized ones is what I use.
I throw the sack in deep water and let it do its thing.

It probably takes me 10_15 minutes to to make.

You can get sandbags on Amazon

Long chain...
3 0r 4 swivels

So your trapping out of a boat. Here's what I did when trapping beaver on some major rivers. I used the stranded steel fence posts. 2 feet of water is enough in most cases. Then run chain from the fence post to your trap location. Here's the most important part of the set up. You need to add some weight to the trap chain, and it should be close to the trap. The first thing a beaver does is head for deep water. And it can't swim for long with that trap and weight on its foot. I like to target a front foot catch since it takes far less water depth to drown the beaver. I used this system because back in the day we didn't body grip traps.

An old timer I trapped with years ago used the weight and a tangle stake with his 3 and 4 .footholds for beaver and otter..Worked good and was a common method back in the day.
Illegal set here now not considered a positive drowning method.
Back in the day the old timers would use a small bear trap like a number 50 and remove one spring for this type of set.It would allow them to dispense with the added weight That is why when you find an old trappers cabin back in the bush you will find a half dozen small beartraps or wolftraps with only one spring-they were used for beaver.

Beaver Caught "10' long chain" With 4 swivels... They're always waiting on the bank for you. But they will dive in.
It's the way I shallow trap, beaver and creeks