Which isomers (cis or trans) are more water soluble?
That's not quite correct and can be quite easily to be shown as wrong by looking up data tables and/or MSDS. Fumaric acid (the common name of the trans-isomer) is less soluble, with a solubility in water of 4.9 g/L whereas maleic acid (the cis-isomer) has a solubility of 478.8 g/L.

Maleic anhydride would also react in water to form maleic acid.