I have touted this product before, here in the forums, but I will repeat the suggestion since I do believe that there is merit.
3M makes a Marine grade PolyUrethane adhesive sealant for boating enthusiasts. The product model/type or part # is 5200.
It can be purchased very reasonably priced from Jamestown Distributors over the net.
Comes in at least three colors that I am aware of: White, tan or black.
Designed for marine environments (fresh or salt water) above or below the waterline. Remains flexible like silicone and it forms a high strength / permanent bond to many surfaces.
Most all plastic drums, barrels and stock tanks are made from HDPE (High Density PolyEthylene). When I first began DIY'ing my filter system for my fish-bait pond (tank) I gave this stuff a try. It is one of the top adhesives for this application that is reasonable to purchase, readily available, fairly easy to work with and has proved itself to me. I recommend it highly.
Note: 3M does not state that it is recommended for use with HDPE. It is my own observation and conclusion that it works.
I can not attest to its comparison with the PL series roofing and sealing caulks with first hand knowledge, but they are also PolyUrethane based adhesives and sealants, so they are obviously very similar. Especially since both are recommended for adhesion to wood or other porous and semi-porous surfaces. But, I do know that the 3M 5200 works very well with HDPE from first hand experience.
The 3M 5200 offers one advantage over the PL series products that I can see. It IS recommended for below water-line applications.
Catfishnut