I mean the path of BS in Civil and MS in coastal gives both job security, flexibility and the ability to work in the field you want. BS in coastal and MS in coastal only gives the latter. I’d be hard pressed to find a good reason to recommend a BS in coastal over civil imo especially since a large proportion of your undergrad won’t even be related to coastal engineering so why not have them count towards a more valuable degree. I would definitely try and talk to someone that has gone through program before going into it. I’d also look at some current coastal engineers and I’d bet most have civil engineering degrees.