Begin with random choices of squares to sink ships. I especially like the spots one space diagonally in from the top-left corner, if that misses I may hit each other corner one space diagonally in (until one hits).

Beware of repeating any opening salvo, or strategy, with repeat opponents - it will be anticipated and defeated. That said, here are some ideas for sinking ships:

Quadrant and Edges Opening Salvo: plant a salvo in the middle of each quadrant, then survey the edges and corners. Many players will follow a missed salvo with subsequent salvos to adjacent squares, some follow with salvos to diagonal squares. Following a pattern may help cover wide areas of the board efficiently at the outset of the game. Diagonal salvos may help create shorter channels to isolate smaller ships. Once you have a lot of missed salvos on the board you can switch to the intersecting channel strategy to find open channels where the longest enemy ships may be lurking.

Intersecting Channels: once you’ve laid down a few misses look for intersecting channels without any misses or previously located ships. At least one of these intersecting channels should be long enough to hide your enemy’s largest undetected target ship(s). Try a salvo on the square at the intersection of these two channels. This strategy works best for finding longer hidden ships, and isn’t applicable for finding the smallest ship (the sub). The intersecting channel strategy won’t always find all the longer ships unless you also search for ships hiding in single or parallel channels the length of the longest remaining ship(s). It is just more efficient to look first for two intersecting channels that can hide the longest ship still afloat.

Towards the end of the game, you’re both probably looking for the smallest ships, let your intuition take over at this point, your subconscious if you will. It’s intuitive to space your ships evenly. Your ‘mind’s-eye’, or your sub-conscious, is really good at this sort of thing, let it tell you where it thinks a small ship belongs. Turns out that it is excellent at this chore. Go with it!

Ideas for placing your ships;

Random: ships are placed randomly, without regard to separation between ships and shore. Randomly placed ships can be difficult to detect where there isn’t a regular pattern for your opponent to anticipate.

Pattern: the ships are placed in some regular (not random) pattern. They may be placed symmetrically, in parallel, or perpendicular to each other making a logical pattern - this suggests the pattern could easily be anticipated, revealed and used against you.

Edges: place the ships such that their ends are on, or along, the edge(s) of the grid. The converse would be to avoid the edges altogether, or to place your ships one square away from the edge, but parallel to it. This approach may include either avoiding the corners, or placing ships in the corner. Using the edges, and especially the corners, could lead to easy detection once the edge placements are anticipated or revealed.

Spaced: arrange ships such that none are touching, or none are within two squares of each other. This placement might help avoid some hits from opponents searching for the direction to follow after an initial hit.

Grouped: this is the converse strategy to the spacing strategy, meaning that at least some ships are placed adjacent (and touching) other ships. This anticipates that once a ship is sunk, the adjacent squares tend to be avoided. Once the initial hit on a ship is made, a player will then search the other adjacent squares looking to sink the revealed ship. If you placed ships adjacent, the opponent may inadvertently hit your adjacent ship. Regardless, this approach is used because the squares adjacent to a sunken ship are often ignored and passed-over. The frequent placement of ships next to each other (adjacent) shows the popularity of this approach for placing ships.