"The lake is deep."
jaQ simply says that you think the depth qualifies as being "deep" relative to other things which might be more "shallow" (jaQHa'). The sea is jaQ, but a pool is jaQHa'.
Saw' is used to indicate exactly how "deep" it is and can be used to measure things that are both jaQ and things that are jaQHa'. vaghvatlh chorghHut 'uj'a' Saw' bIQ'a' and loS 'uj Saw' QalmeH DoQmIv'a'.