My D&D campaign having taken a turn towards the seas, I tought it would be fitting to have ship plans - at least the exterior decks. Unfortunately, I couldn't find any color ship plans, much less multiple standardized ones.
So I went surfing for as much historical ship information as I could find. I settled for the pre-galleon-dominance era so as to have as much diversity as possible. The main ship types I used are the cog (bulky, slow, 1-square-sail ship designed for cargo), the caravel (sleek and fast), the carack (outrageous castles, kind of a galleon caricature) and the early galleon. I rounded those with smaller boats.
I was using Gametable at the time, so each square has a 64-pixel side, same as my existing maps and creature icons (I later found MapTool). Having just upgraded from MapX to Photoshop Elements, I decided I'd go for high-resolution textures instead of pixellated colors.
For consistency and ease of use, I decided against directional shadows. My ship plans have to make sense whichever way they face. Instead, shadows diffuse all around according to height.
Also, I'm a DM first, so I tend to prioritize ease of use instead of artistic merit. I make sure every square is clearly defined so that players never have to guess whether they're on the ship or in the water. All ship contours are as close to their real-life inspiration as I could make them while still fitting the squares well.
I also designed two other ship types for a nation that had some kind of magical-industrial revolution. Y'know.
Apart from the free textures found on the web, everything was done more or less automatically in Photoshop Elements. Ropes were drawn using multiple lines and shadows by abusing "Feather Selection". I strayed as far as I could from my semi-pixel-artist Paint roots and drew every element using shapes and tools, usually creating similar parts of every ship simultaneously. Also, the ships are surrounded by their own shadow and paler water, so they should feel at home on most maps (or water textures).
But enough talk for now.