A hugely complicated subject - here is a simply explanation!
There are two functions for the diodes - Blocking Diodes that stop the batteries discharging into the panel at night - and Bypass Diodes that help minimise the effects of shading.
Most large panels
don't have Blocking Diodes because a charge controller does the same job.
The more Bypass Diodes in a panel the better - they 'bypass' cells that have a shadow that would otherwise block the current
and reduce the output. Ideally all cells would have them but for ease of manufacture 'groups' of cells have a Bypass diode.
It starts getting complicated, and controversial, when you put panels in series or parallel to make a large array and feed an MPPT
controller. Many other threads on here and elsewhere arguing which is better - have panels in series to reduce losses in the cables
and have a higher voltage at the input to the controller, or have them all in parallel and reduce the effects of shading.
The bottom line is all depends on your panels and how many Bypass Diodes they have - do some long-term tests and see which works best for you on your boat - Series or Parallel!