Plain is never completely plain
A patch of water that looks flat at a glance almost always changes slowly in brightness. Underwater, light comes from the surface, so the blue is paler at the top and darker below. In the sky it is the reverse: pale near the horizon, deeper overhead. Sand under shallow water carries a net of bright lines where the ripples focus the sunlight.
Before you start on a plain area, take a moment to look at the preview and notice which way the gradient runs. That single observation turns a heap of identical pieces into a sequence.
Jigsaw: sort first, then join
Pull all the plain pieces into one part of the table and arrange them by brightness, lightest to darkest. Border pieces go into their own row. Then work along the gradient: the lightest pieces belong near the surface or the horizon, and you only have to try a few candidates for each gap.
When colour stops helping, use shape. Each piece has four sides, each with a tab, a blank or a straight edge. Look at a gap, count the tabs it needs, and pick only pieces with that pattern.
Turn puzzle: follow the light
A plain tile can look right at two angles. Decide using the gradient: the brighter side of a water tile is usually up. In sand, follow the light lines across the neighbouring tiles. If neither helps, finish all the tiles around it first; the last tile in a gap has only one direction that continues its neighbours.
Leave it for last
In every game it pays to finish the detailed parts first. When only the plain area remains, every placed neighbour narrows the choice. The dive plans on this site name the plain area of each picture and measure how much of the frame it covers, so you can see in advance where the slow part will be.