Which signal should i be using

This occurs because it takes time to load: proportional to the amount you are loading. When the first ship loads up, it leaves less for the second one to load, causing the second ship to spend less time in the dock, and therefore catch up to the first. As the second catches up, there continues to be less and less for it to load, causing an accelerating cycle that ends up in the state you have pictured.

The best way I have found to combat this is to make all (or all but one) of your vehicles wait until fully loaded. This causes less available cargo at the dock to mean more time at the dock. This reverses the cycle and causes your vehicles to remain relatively well spaced.

The a single vehicle on the route not fully loading will not cause this problem, as it will eventually catch and pass the fully loading vehicles ahead of it. Such a vehicle can be a good indicator of when you need to add or remove vehicles.

One-way signals reserve a signal block between it and the next signal.

One-way path signals reserve an individual path between it and the next signal. This allows 2 trains to be on the same signal block at the same time, so long as they have paths that don't cross.

From the wiki:

The two new signal types behave a bit differently than standard OpenTTD signal types. The Path signals are red by default, and will only show green as soon as a train can reserve a path to the next safe waiting position on its route. Safe waiting positions are - by definition - in front of signals, depots and track ends. The back of a Path signal is not considered a safe waiting position, and therefore paths are reserved through these signals.

Because the front of every signal is defined as a safe waiting position, you would normally not want to place a signal immediately behind a junction, only in front of a junction. This is because it is only safe for a train to wait in front of a junction. It is not safe for a a train to wait at a signal immediately after a junction before the whole train has cleared the junction, as it would be blocking the junction while waiting, as illustrated in the example below. This is a major advantage against standard OpenTTD signals, where you had to place signals before and after junctions, which caused trains to block junctions while waiting.

For straight track and basic merges/splits, I stick to normal one-way signals. I use path signals for forks that have 4 or more tracks, as this is where they start to benefit.

Path signals also can behave similar to entry/exit signals, although entry/exit signals have uses that path can't fully achieve. For instance, entry/exit signals will cause trains to take longer routes if a path is blocked, while path signals may wait. This can be used to construct balancers and other helpful constructs.