In Catan Explorers and Pirates, can a ship share a hex side with a road?
Yes, 2 ships and a road can share a coastline.
According to the FAQ for Catan Pirates and Explorers:
Sea Routes - Are only ships allowed on "sea routes" located between terrain hexes and sea hexes / frame, or may I also build roads there?
Roads may be built on all paths, both inland and along the coast. At the shore, roads can be parallel to ships, meaning that up to 1 road + 2 ships may simultaneously occupy a path on the coast.
Source : Catan FAQ page 157
Looking at rules [here] I see no reason why not1 Small Caveat, I've never played this expansion but the rules seem fairly clear.
Firstly I can't find anything in the rules that prohibits this. If this was not allowed I believe it would be explicitly stated. The rules do include very details rules about the building a movement of ships. The following all make me think its fine as well
The rules define a Sea Route as :-
The edges of a sea hex are called sea routes. Sea routes separate sea hexes from each other and terrain hexes from sea hexes.
So if the ship is in a sea route than it cant be in same space as the road which is on land.
In the rules for building ships it states :-
When you build a ship, place it on a sea route directly adjacent to one of your harbour settlements, with the following conditions:
None of the following conditions say that you cant build a ship a sea route next to a road.