Depends on the plastic and in particular, what filler is used. Some can be less dense than wood. An advantage of plastic is that it can be formed, or welded into shapes that give it the most strength where needed. Suitable buttresses on the outside can resist the weight of water it is holding. You need to look at stiffness, yield stresses as well, when compared to wood. A plastic stop plank won't look the same as a wooden one, but could be equally, or more effective. Welded steel stop planks might even work. Shaped and buttressed to minimise weight, while giving enough strength and stiffness. Could even have rubber seals at the joints and against the slots. You need to look at lifting gear, especially if the Marina has an entrance sized for fat boats. The Bridgewater canal has fixed cranes on site at many of their stop plank locations, ready to deploy the in situ wood planks quickly, but, as @Cheshire cat says, it all depends if the system actually works on the day. On CaRT waters, you can spot eyelets under bridge arches occasionally, once used to lower stop planks off a block and tackle. Modern lifting regs mean they are almost certainly no longer used and CaRT bring in a properly tested and certified crane. The marina will have an ongoing cost on their lifting gear, maintaining it and keeping the paperwork up to date.
