BaalHabooze
I think the answer for Beshalach q is that the makos were punishments on Mitzrayim so they deserved that. Krias yam suf was not a punishment (at that point) but rather to allow klal yisroel to pass through. They were undeserving of a neis of that magnitude because it was for them and they were also oivdei avodah zora.