No, the assembled scuba goes into the tank well behind the seat. The spare tank (second tank) goes in my forward hatch (Scupper Pro TW). My other equipemnt is stowed, tied, bungeed, clipped or worn.
BC inflated and regs pressurized which means air full on! Clipped off in the back of the kayak, and strapped down. I do not think I could swap an HP100 on the water without major issues, like water into the first stage. So we never carry a spare tank. Instead we just head back to our launching point, which is typically the only safe area to come ashore.
I find kayaks to be great for simple warm water diving. I know people dive with them dry but you will have to get one of them to explain their process, I cannot.
Our coast is very rugged with only a couple of launching points for boats, but there are a few if using kayaks. Drysuits are required here for diving warm. I usually just get out a ways and bail off the kayak to cool off, then continue on our journey out on the ocean. I put my dry gloves on before departing shore, then doff everything else once anchored.
My flag and float and a 50 foot tag line go in the tank well until anchored (or if about to go in if drifting or towing). The dive flag is the smaller type, in Florida if the flag is on a boat it has the be the larger size, towed it can be the smaller size.
We fly a flag off the kayak, but it is impossible to use a float in the kelp, or tether the kayak for the same reason. The kayak dive flag is stowed under a bungee on the port side. The paddle is stowed on the starboard side.
I carry an anchor always and it is a small 4 pound folding grapple and I usually have several bundles of 1/4 inch nylon braided line with clips for anchor or tow duty.
Due to our sometimes extreme conditions. I carry a 5 lb anchor and have a few feet of heavy chain in front of it, with 200 feet of line on a cheesy reel. Overkill? Probably, but we tie two kayaks together with a bungee cord I made, and it is important for us to make it back to the kayaks and our launching point safely. We can anchor in over 100 feet of water with no issues. A depth finder is a requirement here.
I have installed "pool noodles" and two air bags inside my hull so that should I flood the boat (roll it with a hatch open) the boat will remain afloat. Warning, anything you value, tie it down, clip it off or you will doante it to Davy Jone's Locker. My paddle gets a bungee as well. My lifejacket goes up front ahead of the forward hatch unless conditions are bad--then I wear it.
My kayak was filled with expandable foam between the hatches. It is unsinkable, and nothing can get lost in the caverns below, as the foam creates a barrier. I always know my mask and other small gear will be close by.
Cary a sponge and a bilge pump, VHF, cell phone, signal and rescue gear, mirror, flares etc.
I carry VHF, flares, bilge pump, cellular also.
This is an opinion based on my experience, each diver should have their own yak, one diver per yak, two seats jobs are hard to paddle solo and don't carry enough gear for two.
N
Two seaters that can be paddled by one person work pretty well for kayak diving as I have witnessed.