I simply have a strong line with brass clips at each end. One end is clipped to the boat...the other to the stainless gear ring on my BC. Then I strap everything down really well.
When I reach my dive site, I unstrap the gear, and throw it overboard. It is still attached to the boat until I leave the boat, don the gear, unclip, and descend.
No need for the brass ring because you have the gear loop on your BC.
Exactly what I do for the tethers. The rings on the tanks are for taking two tanks out (no sense paddling all the way back in for a second tank against wind/current). A knot around the valve is apt to work loose so I just clip them off and drop them in the water rather than let them sit on the kayak. I wouldn't trust a tank just strapped in if the kayak goes over in a wave, or tethered with a rope around the valve in any kind of significant seastate. Also, I have an old Dacor that is black and I don't like to let it sit in the sun, although it might be kind of funny to see a jet propelled kayak when the burst disk blows! If you're only diving single tanks then the tank's always with you and the point is moot.
Anyway, it was just a suggestion. Don't make it too much work or it isn't fun anymore. Heck, enjoy it snorkeling for that matter - just a paddle, pfd, sunscreen and snorkel gear!
I'll see you around this season Jenny, it's been too long!
Steve