I have five "Tag Lines" on my little SIB. The key to remember is to throw them overboard BEFORE getting off the boat
The length should vary to adapt to your tie off point and the width of your tubes. I like them long enough to have plenty of room to stay away from a bouncing boat as I take my gear off. Mine are around 6-7ft, so a little over 2 meters. You will need longer ones.
Since I want them to have give instead of yanking on my gear hard during rougher conditions, I made them with a 3/8" bungee cord inside. For strength so I don't loose gear, I make them out of 1" tubular webbing. I sew bolt snaps on both ends (Found them to work great as dock bungees sometimes too). There is no fear of the webbing breaking and the sewing is strong enough I trust my life to the climbing gear I make. A #18 needle on any decent sewing machine does the trick. The weakest link is the bolt snap (Errr saw handle, see photo below).
For my gal I put a d-ring on her weight belt, locked on with a clip. That way when she returns to the boat she can connect a bolt snap to the belt, remove her waist and crotch strap, then drop the belt before doffing her BP/W. My tubes are low enough, I just toss my belt into the boat. We clip the tag line off to our left chest d-ring before doffing our BP/W's. Not sidemount, so your tanks will probably sink. I would recommend keeping them close to the more stable stern.
I don't have a photo of one of my exact tag lines handy, but this is a lanyard I made for my top handle arborist saw, which is very similar. Just exchange the carabiner for a bolt snap. (No need for the red or blue part is that is my safety breakaway, so I don't get drug out of a tree. Didn't get used last week though when the saw got yanked out of my hand. Wood piece is still stuck in the chain, but it tore the front of the handle off the saw.) The 3/8" bungee cord is feed inside the tubing, then tied with just an overhand aka half hitch knot. If someone wanted to get fancy they could sew it. I just set them up to have a foot of give or so.
Careful with poly as it is hard to tie knots in, and can slip. A couple of zip ties can stop it from coming apart. Poly is what my boat has for lifelines, and I had made a couple of tag lines that way but retired them after noticing a little slippage.
Of course some rope or webbing and a couple of bolt snaps using a bowline or buntline hitch would work. If you wanted to add in some loops use an Alpine Loop and you could tie some bolt snaps in there too. No need to sew, even if you wanted bungee cord inside tubing.