I can't quite use the random spawn command

soooo i cant make my mobs DON'T spawn under water i looked it up on youtube and google but no use the only guy i found was useing a costume world generator and the commands he gave didint work like "- block{b=water} false" or something like that i dont remember the same command rn but you get the point can some one help me im new to this plugin and i love it aaaand i want to learn it thx :)