yeah its a dupe for sandbox xD its not a addon :D
Just a Dupe :)

i cant find the landing craft to spawn, is it only on sandbox?

press q > than look right there is a list of tools > click there on "physical properties" > after u clicked it it should be at the right side right near the tool-list a litlle menu > in the menu is a field to change the physical property click on the field > choose wood > close the q-menu > take the "toolgun" and click on each part of the ship > after it should swim in water > if not write back than i going to make a fixed version
:D

sorry im new to gmod how do you activate the physical property tool

u need to take the "physical propertie" tool
Than u choose "wood"
After u take the tool and spam (on nearly every prop) of the landing craft

Than it shoould work if not say i try to reupload a bfixed version
;)

i dont get what you mean by making the wood a physical property