you can only build a pyre (current version has workshop art) on living resources. Things like cow, sheep, pigs, horses, corn, rice, wheat, dye, etc. If you download my D'tesh Water module you will also be able to build another improvement on water tiles containing living things like kelp, fish, clam, shrimp, and crab which is called the Cremation Boats. They work like pyres, but on water. To build them you need to have the D'tesh Arcane Barge UU (also in the module).