I'm still doing my trimming on my ponds with a weed whacker.
I had a DR string trimmer/mower, with the offset feature, but it didn't do much better than a regular lawn mower. To get close to the water, I still ended up with the mower in the water. Or, I had to go back and forth. Thus, the weed whacker was quicker and less work.
My brushhog is about 6-inches wider than the outside my tractor tires, but it is just to dangerous to use close to the deep/steep edges of my pond.
I keep thinking about a getting another sicklebar mower. Unfortunately, everytime I think about one, I think about my friends and relatives with less than 10 fingers who regularly worked on them. I grew up with them. They do take a reasonable amount of maintenance, but they do a great job of off-set mowing.
With most 25-50 hp Cat 1 tractors, the 7-foot arm will cut at least a foot beyond the outside width of the tires,but by adjusting the arms of the three point hitch, you can get nearly two feet of cutting beyond the tires -- but the hitch has to be kept very tight to prevent nicking or cutting the tires.
As old as they might be, my favorite has always been the Ford 501. A gazillion of them were built. They are just plain tough, and parts are still readily available.
At auction, I see them regularly sell for between $100 and $500.
