"They would like water!"
The conditional requires a conditional clause and a main clause. "If there is nothing else to drink (cond.), they would like water (main)".
Since there is no conditional clause, conditional tense does not work here.
ETA: Just realised my example would still be translated as -omba, not the -penda used in your sentence. How about "if they were able to swim, they would like water," which I believe is wangeweza kuogelea, wangependa maji.