I had an Albacore and it was a great boat with few problems. So I purchased a Marlin and it was a pain. When I built the Albacore I used the supplied WTC system which worked fine, but I had nothing but trouble with the Marlin WTC. The pump did not work well and the T valve leaked pressure constantly. The issue with the T valve may have been my fault as I broke it somehow trying to install it and I may not have repaired it correctly. The pump appears to have had the pump vanes slip on the pump motor shaft which allows the pump motor to spin and not move the pump vanes. Also I had a lot of trouble with the position of the pump and ballast tank within the WTC. Even though I checked the position before I glued it the position conflicted with the center brace which holds down the top of the WTC.
Mind you that most of my trouble with the WTC came from my gluing the pump and tank in the wrong position and too far apart on the piece that joins the pump to the tank. To fix my poorly located pump and ballast tank what I did do was cut out the whole pump and tank from the lower WTC and move it forward 1/4 inch and install a "spacer: to correct the separation between the pump and ballast tank. This helped the lid fit on but did nothing to fix the pump and T valve not working correctly. I checked the instructions and everything was routed/installed correctly.
Now for the learned things. One thing I would recommend is to cut a hole into the top of the ballast tank. Then simply use RTV glue to re-secure the top back on. The reason I say to do this is that in the future the hose material used in the tank will deteriorate and not allow the tank to fill or drain. If you have access you can check it. Also I would use a piece of brass tube and make it a schnorkle that comes out of the top of the sail. Add a periscope and the model will be complete. These two items will help the model surface and run at periscope level. You will be better able to see the submerged model and have a bit of extra reach when you want to surface with the schnorkel.
I am currently working on a new WTC from Atomic Subs with a gas ballast system. This may work out much better than the offered WTC system if pump and tank. We will see.
But I would recommend the Marlin or Albacore to anyone interested. They are good kits and most modelers can build them with little trouble. I did have the thought that my trouble with the Marlin may have been due to the fact that I thought I knew what I was doing. It seems like every time I have that thought I have one right after that thought that goes something like this "Boy, I should have read the instructions first" haha!
Otherwise I would like to recommend the kit supplier have or be more flexible with spare parts. If memory serves me the owner is now in England and the US supplier is in the US and he was not sure at the time if he or how he could obtain spare parts. This may have been due to the fact that he was just receiving the kits and the change from Skip to the new owner had just occurred.
Please understand this is not a slam on the kits or supplier only my attempt to inform those out there so they can benefit from my experience.
I am currently contemplating a second Marlin build. If you have any questions let me know and I will try to help you.