The offer should be pending a satisfactory survey to begin with. If not satisfactory, then renegotiate over those items, etc. Your insurance company will probably ask for one anyway. Even if it doesn’t, the marina you take it to might ask for an out-of-water survey if the boat is 20 or more years old.
But, to answer your question, look for evidence of water ingress from the deck/hatches/port lights for one thing. Rust stains/streaks around metal (e.g., chain plates, under stanchion bases), discoloration of wood, sometimes strong mold/mildew odor in dark places, etc. (Fresh water can run down the hull from a deck leak and accumulate in low places). Deck leaks are a major PIA.