If you want to utilize your dock as an outdoor space, you have to embrace three key design challenges: Small footprint, weight restrictions and impermanence.
A small space is a challenge to design for regardless of the location but couple in the utilitarian nature of a dock and designing for this small space gets even trickier. Regardless of the space when starting a design plan I always start with the basic question: How are you going to use the space? Single use requirements are of course much simpler to design for than multi-use, but if multi-use is your answer then start looking at furnishings through the lens of multi-purpose. When shopping ask yourself, "How many different purposes can this one piece serve?" Ottomans are a great multi-purpose furnishing to use as an example. When an ottoman is added to a chair the chair becomes a makeshift chaise lounge. Add several ottomans to a sofa and the sofa becomes a daybed. Add a tray to the top of an ottoman and it becomes a side table. Push several ottomans together and you create a makeshift lounge height dining table. Used alone ottomans become individual seating.
As if a small footprint wasn't enough of a design constraint you will also need to consider the total weight and placement of the furnishings you are adding to your dock. This design challenge will also push you to consider multi-purpose furnishings. The more uses one piece of furniture can accommodate the less furnishing on the dock and the less weight. When ordering furnishings be sure to keep a weight total of the items that you are purchasing. You will want to know the total weight of furnishings for the dock and you will also want to keep track of the individual weights of each of the pieces and where the heaviest items will be placed on the dock. You won't want all of your heaviest furnishing on one side, for example. All retailers can tell you how much a piece weighs if it isn't already listed in the catalog or on the website.
Most outdoor spaces are impermanent - lasting only a season or two, if you are lucky. The lifespan of a dock space could be as short as a weekend if it's a weekend home. Here again multi-purpose furnishings and weight plays an important factor - the more uses one piece of furniture can get the fewer pieces in your grouping and the less to carry and store. If you are going to be moving furnishings from a storage shed or boat house to the dock, you'll want to be able to do it with ease or you will find you don't want to do it at all. Think about how easy furnishings will be to move and to store. Take a close look at how furnishing will stack or nest.