I've heard you can get "bass coupling" by putting the mid-bass closer together, maybe someone who knows what than mean can explain. I like them out as much as possible so it leave headroom in the "walkway" down the middle of the boat.
I find my tower speakers are very directional. My speakers rotate in the housing so if I line them straight back and stand right behind them vs. 3 feet to left or right of swim deck the difference is hugh. I would suggest spreading them as far apart and if possible pointing them to each corner of the boat to spread the sound as wide as possible instead straight back