I recently found a diver that seems to work pretty well, although I have only had a limited chance to use it so the jury is still out on overall performance. It is an adjustable diver made by some company called Davis Instruments and can be set for up to 80ft of depth in 5ft incriments. It also rolls over and comes to the surface upon a fish striking it, supposedly. I have only used it fairly shallow as that is where the fish that I was targeting were holding so I don't really know the skinny on fishing deeper, only what their claim is. So far I've been impressed with this diver, more so than any other diver that I've used, but only for fish under 24 inches. Bigger than that and I think it might be too hard on the diver. But this diver has a very limited drag compared to other divers I've used and is much more pleasant to use with light tackle.
As far as not being able to mount a downrigger on an inflatable, that is only your choice. It is quite easy and innexpesive to build a reasonable rowing frame from EMT that is comparable to what is in most river rafts. It may be a simple frame unless you have significant skills but even a simple frame will multiply the performance you can get from your inflatable. I too fish from an inflatable and I love it, but largely because it has a frame which enables me to mount rod holders and fish finders and the such. Mounting a downrigger to your boat would not be that complicated. I don't do that because I don't ordinarily troll for fish and so I don't want it in my way. In a small boat, it doesn't take much for excess gear to get in your way. But if I were to troll a lot, I would not hesitate to install a downrigger on my inflatable, and it wouldn't be one of the cheesy tiny little ones either.
As far as fishing depth with weights, this can be done successfully but you will not necessarily know the depth at which you are reaching fish. You can let your line out to a certain number of pulls and then just keep increasing until you hit fish. As long as you've kept track of how many pulls of line, or how many feet if you have a line counter on your reel, and you know the amount of weight you were using, you will know exactly how to reach that depth again and you can be quite precise about it. And line counters are a pretty cheap add to your rod and will make you much more precise in trolling this way. The big problem with fishing with lead is it takes most all of the fun out of fighting the fish. You'll find you are just dragging them up to the boat to land them. When you can feel that you have them that is.
If it were me, and I wanted to troll for fish, I would definitely install a downrigger on my inflatable. But my second choice, which is the one that I currently use, is a Davis Instruments diver. When I fish for kokes it's usually earlier than this and it is mostly all surface action using jigs. Of the jigs I use, they include, Buzz Bombs, Zingers, Nordics, Gibbs Minnows, P-Line, Tasmanian D's, Crippled Herring and some others custom made jigs that are sold out there. The Gibbs are my favorites and I use them mostly in 1/2 and 1/4 ounce models in either pink or orange, and preferably with some white on them for contrast. If the wind is blowing hard or I need to fish down deep, I will sometimes move up to a 3/4 ounce jig. I usually do not jig for the bottom dwellers but I often will jig for the ones that are suspended between 30 and 70 feet or so. I don't jig for the bottom dwellers because I don't do well with them and I'm usually doing quite well when staying up shallower. I have witnessed others who do very well when jigging on the bottom but they usually are tipping their hooks with some corn or maggots or something. I've found that when jigging in the methods that I prefer, tipping the hooks actually gets me fewer fish than when I don't. I can't speak for everyone but this is what my results have been.
Good luck to you in however you go about it.
