"Do not immerse fish directly in water" is advice often given in the context of storing, thawing, or preparing fish. Here are some common reasons and best practices:

1. Maintaining Texture and Flavor
Leaching of Flavor: Prolonged contact with plain water can cause the fish’s natural flavors and some of its nutrients to leach out.
Mushy Texture: Fish flesh is delicate; exposing it to water for too long can make it absorb excess moisture, leading to a soft or waterlogged texture when cooked.
Best Practice
Sealed Packaging: If you must thaw fish in water, place it in a leak-proof plastic bag first. This keeps water out while still allowing the fish to thaw.
2. Food Safety Considerations
Bacterial Growth: Leaving fish uncovered in water at room temperature can accelerate bacterial growth, which can lead to foodborne illness.
Cross-Contamination: If fish comes into direct contact with water in a sink or container used for other foods, you risk contamination—especially if you’re reusing that container without thorough cleaning.
Best Practice
Cold Water Thawing Method: Submerge the sealed fish in cold water. Change the water every 20–30 minutes to keep it sufficiently cold and safe from bacterial growth.
Refrigerator Thawing: Whenever possible, thaw fish overnight in the refrigerator in a sealed container to keep it at a safe temperature.
3. Proper Cleaning vs. Soaking
Rinsing vs. Soaking: Rinsing fresh fish under cold running water to remove surface contaminants is typically fine (and sometimes recommended), but soaking or immersing it for extended periods in water can degrade quality.
Drying: After a quick rinse, pat the fish dry with clean paper towels. This helps achieve better browning or searing when cooking and preserves texture.
4. Storing Fresh Fish on Ice
Ice Layer Separation: Some fish markets and home cooks store fresh fish on a bed of ice without immersing it. This method keeps the fish cold without drenching it.
Drainage: If you’re using ice at home, keep the fish above any melting water. For instance, place a colander or perforated tray over a bowl or tray lined with ice so the fish stays chilled but doesn’t sit in water.
5. Additional Tips
Freezer Storage: For long-term storage, wrap fish tightly in airtight packaging or vacuum-sealed bags to prevent freezer burn and flavor degradation.
Quick Thaw Option: If time is short, use the cold-water method (in a sealed bag) or a microwave on the defrost setting—being mindful that microwaving can start cooking the edges.
Safe Temperatures: Cook fresh fish to an internal temperature of 145°F (63°C) to ensure it’s safe to eat (or follow local guidelines for fish consumption).
Bottom Line

“Do not immerse fish directly in water” typically means: avoid soaking fish in water, especially when thawing or storing, because it can deteriorate the fish’s flavor, texture, and safety. Instead, use sealed packaging or place fish above melting ice or water to preserve its quality.