Hey, Kyōhei, this problem... it's the saltwater one, but...
Literal
Hey Kyōhei, this problem... salt-water [gen-の] one [copula-なんだけど]...
あのさ is a casual, hesitant conversation-opener — 'hey / um.' 食塩水のやつ 'the saltwater one' uses やつ as a casual pronoun-like reference ('that thing / that one'). The double trailing off (・・・なんだけど・・・) creates a halting, embarrassed rhythm — a student trying to ask for help without directly asking.