my water tanks are actually the port and starboard settees. each holds about 75 gallons. each has an opening port you can stick your hand in mounted atop a removable plate about 14 inches square that's held down by two dozen screws. makes it really easy to do an annual cleanup of the tank interior. my starboard tank developed a leak somewhere that i could not find so i just removed the cover plate and then sanded, cleaned and sealed the whole interior of the tank using gluvit. end of leak.

i installed a holding tank a few years ago (1979 boats didn't come with holding tanks). there is a rather large storage space under the pullman berth in the sleeping cabin (this is a csy 37 with the B plan interior) about eight feet from the head. was easy to put in and would be easy to take out, but i had to give up some good dead storage space.

the stainless steel diesel tank is located in the salon, partly under the dining table. access is by an easily removable section of cabin sole. there is a bolt that unscrews giving me a 3/4" hole to insert the 'fuel guage' - a 16" wooden dowel marked in 5 gallon increments. no electric fuel guage for me! there is also a square ten inch stainless plate held down by screws on the top of the tank. removing this gives me access to the inside of the fuel tank for servicing the pickup tube or cleaning the tank. and that's about the only problem in the whole tank saga; because the tank has two interior baffles i can only fully clean the center section of the tank. it would be possible to remove the entire tank by taking up the sole but i haven't ever considered doing that.