Build a fleet without a recognizable pattern
Mix horizontal and vertical ships as well as central and outer positions. If every ship follows one system, the first hits teach your opponent to search your habits instead of the whole board.
An edge ship may survive the opening search, but a hit leaves fewer directions to test. Use the perimeter for one or two ships rather than the entire fleet. Place long ships in different regions first, then use short ships to fill less obvious spaces.
Use a grid while hunting
With no active hit, your task is finding a new ship. When every surviving ship occupies at least two cells, a checkerboard search intersects every valid placement without wasting adjacent shots.
Review the remaining ship lengths after every sinking. Do not shoot into a continuous gap too short for the smallest ship. Prefer cells crossed by many still-valid placements.
Determine direction after a hit
The first hit opens up to four directions. A second connected hit reveals the axis; only the two endpoints matter after that. If one endpoint misses, switch immediately to the other.
Do not abandon a clear target for an unrelated guess. Return to the search grid after the ship sinks and apply every new exclusion. Each shot should finish a ship or meaningfully reduce the search space.
Play against your own habits in a rematch
Previous games are information when you play a friend. Repeating a successful layout teaches your preferred corners, orientations and spacing. Moving even two ships can invalidate those assumptions.
Vary the style as well as the exact cells. Follow an edge-heavy game with central placements and replace mostly horizontal ships with mixed directions. The goal is not a perfect layout but one the opponent cannot know in advance.