The problem with crusades is just that. Most of the time half the army ends up deserting before you even get tot the location.

To counter this, of course, you can always hire mercenaries which are, let's face it, just terrible. So, the best strategy would be to attack places close to home, or get to the location as quickly as possible, and this is where fleets come in handy.

If you are Sicily, for example, the fastest route to the Middle East is by water, and the same holds tru for most if not all western European factions. It isnt as big of a deal if you are in the east already, but fleets are almost always necessary to get there on time.