Flatland skimboards...
So, when looking for a flatland board, you may be better off getting a wooden board as foam boards may snap when used on rails or other objects because of the pinpoint pressure. The best kind of flatland board (personally) is the twin tail skimboard (more shapes are below).
Most flatland boards are made of plywood covered in fiberglass resin to protect it from shells and water seeping in. If the coating of the board is cracked or damaged, you should really repair it otherwise your board will get waterlogged (soak up all the water and weaken) and further damaged. It could make the plys start splitting.

Waveriding skimboards...
When searching for a board for wave riding, make sure your board comes up to your mid-chest. Try find a board suited for your weight as well so that it keeps you up on the surface of the water.
A bigger board will generally go faster and be slightly more buoyant but much harder to maneuver. Check with the board seller which board is right for you. Too much rocker can slow down the board, so make sure you get enough rocker to bounce over the ripples and water mounds and just enough to give you a lil pop (pop is the amount your board jumps up when you ollie or pump your board).

Skimboard Shapes The following are the Flatland and Waveriding skimboard shapes out there. If there is "Flatland/Waveriding" beside it, then it's probably better for flatland unless it is a foamboard construction.
	Flatland With a flat nose and tail this bad boy can offer a more responsive ride, making sure you can line up and hit anything you want!		Flatland A square tail helps give stability and a bit more pop. Possibly the most ideal beginner flatland board.
	Flatland/Waveriding This is rather similar to the teardrop shape perhaps not quite as responsive.		Flatland Much like the pin tail, minus the maneuverability, plus the pop.
	Flatland/Waveriding Also known as the swallow tail. Have a bit more carve and allround skim with this shape.		Flatland Much like the fish tail, minus the maneuverability, plus the pop.
	Flatland/Waveriding This ride is great for the in between waverider and flatlander. Easy maneuverability has great response.		Flatland A good beginner board for flatland. Has nice amount of pop.