The small matter of orientation - that's solar panel orientation

I have been thinking about installing some solar panels.  Now obviously if you are installing them on a building, it is easy to see that mounting the panels to face in some directions would be better than others.  In the Northern hemisphere, pointing the panels in a generally southerly direction is obviously better than a northerly direction.  At a particular time of year it might even be possible to find the optimum angle.

However, on a movable structure - namely a boat - it would be much more difficult to choose the best direction.  On any day of the year the boat might be facing to the North, South, East and West at some time of the day.  That could leave you constantly worrying about setting your panels to produce the largest amount of power.  This is something that I don't want to bother with.

The one direction that you can always guarantee to point to, with reasonable accuracy, is UPWARDS!   So I have been wondering whether this is could be the best compromise.  Obviously it is not the very best in ideal conditions, but from the point of view of a pragmatist who wants a tidy roof and the least possible bother, would this be a silly choice?

It seems not.  According to this site, pointing vertically upwards will lose 10% of the optimum energy collection (in the UK at least).  At lower latitudes that loss would obviously be reduced.  A mere 10% loss is much better than pointing the panels in completely the wrong direction for a lot of the time.

Now the only problem is that England is often rainy.  A flat panel will collect water.  How much shall I tilt the panels to keep them clean and relatively dry?