Portside water tank in 1987 46' post

Patience Grasshopper! The forum is a little slow this time of year. The Post guys are quick to help out and I am sure someone will chime in. Obviously I can't, my antique has the water tanks under the cockpit.

I have the same set up.

Fortunately, mine are not leaking.

There are plywood access panels into each one of the voids.
you may be able to move the tank into another compartment to lift out and replace whole without ripping up the floor.
My guess would be to remove the washing machine through hatch to the aft of the stair hatch, then remove the starboard tank, then slide the empty port tank into the starboard void. Then remove and replace.

Great, I forgot the waste tank was in that forward starboard compartment.
Where is your washing machine and dryer?
Some of the earlier models have a washing machine in the cockpit.

By the way the through hull shut off for my generator is in that port side compartment under a hatch in the floor of the compartment.
I owned the boat for a month before I found it!
Its on my list of "to dos" to relocate it into the ER for easier access.

You put it in the quoted material box for RT46's post. You can see your reply if you click in that box (the box will expand). In case I'm not making myself perfectly clear, here is your reply:
"The washing machine was gone when I bought the boat. The dryer was removed yesterday to make access for the removal of the portside water tank.
I'm OK with the intake for the raw water pump (generator) but the chase for the hoses and wiring going forward is a nightmare.
The room and access is great in the compartment now that the "Dryer" is gone (it didn't work anyway). OK back to the grindstone time to pull the portside tank today."

You put it in the quoted material box for RT46's post. You can see your reply if you click in that box (the box will expand). In case I'm not making myself perfectly clear, here is your reply:
"The washing machine was gone when I bought the boat. The dryer was removed yesterday to make access for the removal of the portside water tank.
I'm OK with the intake for the raw water pump (generator) but the chase for the hoses and wiring going forward is a nightmare.
The room and access is great in the compartment now that the "Dryer" is gone (it didn't work anyway). OK back to the grindstone time to pull the portside tank today."

Sanyo but its nolonger in production.

if your goes under the stairs like mine there are a few compact models that will fit.

Are you NUTS??? Do you want to do that job twice? Do not weld a 20 year old plastic tank, even if the weld holds it will just fail somewhere else. Buy a new tank for both sides while they're out,

20 years is the lifespan of a plastic tank. Welding it won't hold, a month maybe, and it will start leaking again if you can even get it to hold in the first place. Isn't there an off the shelf plastic tank that is the same size? $1500 is the cost of owning a yacht, but to go to the trouble of taking the holding tank down, out of the hole, as well as the other tank. The holding tank part of it is the worst as you can't get it totally empty, can't get the smell out, etc.

My water tanks are under the cockpit, I'm curious what you have in that space behind the fish boxes?

I have empty storage space forward of the fish boxes.
One side has a compressor for the cockpit freezer and some cutouts for 5gallon oil pans. the other side is unused storage space.