Freebooter Miniatures do square planked bases in their accessories section.
Buuut if you do want to make them it's a pretty simple 9 step process....
Although I'm sure you can cut it down a bit for rank and file troops or bulk out some units with a simple sandy shoreline type base.
1)Trim/ whittle down/carve up some coffee stirrers to the desired knackeredness. Try to get cheap stirrers, good ones tend to have little to no grain are too thick and are often waxed.
2)Stick them to a base with Superglue. For larger more 'jetty' type structures get some matchsticks and some slightly thicker balsa wood dowel and square sectioned lengths.
3)Use a small drillbit, or bradel to make holes where nails would go.
4)Add detail elements, crabs, barrels (Renedra miniatures make nice ones), rope (twisted brass picture wire, string), tools, cleats, skulls wearing pirate hats, spare splinters of the stirrers etc
5)Weather with light sand, filler,
GW texture paints or equivalent, superglue+pva mix for algal growth.
6)Prime
7)Paint
8)Slime and wet effects with gloss varnish mixed with inks/washes,
GWs Camo Green is a great colour for this.
9)Use Vallejo or Woodland Scenics water effects for deeper pools, swirl in paint/wash/ink for a seaweedy look.
Now this might sound like a lot of work for multiple bases, I'm not sure how many you're planning on doing, but with a good system you should be able to get an army's worth done to an okay standard within a day minus drying time.
Let me know if any of the steps or elements don't make sense and I can explain them better as this was a bit of a rush job.
Hope it helps.
Edit: I saw this and I thought of you....
