I'm playing the HOMM1 campaign as a Sorceress and I think there's a glitch. In scenario 5, I start with a castle on the river, but it's not letting me build a shipyard! I assume this is a glitch?? Is there any way to fix it?

Not familiar with HOMM I, but I don't think there should be too much difference form HOMM2.

Is it or river or sea terrain?
You should post a screenshot.
To build shipyards, Castle must be next to sea terrain. More specifically - sea terrain should be right next to castle entrance.
If this is the case try to add shipyard to the castle in the editor (if possible). See what happens.

It's next to a river, not the sea. My bad! Thanks for explaining. :-)

Played that one a few weeks ago. Be careful, the knights across the river CAN and WILL build boats to attack you as soon as you go a bit too far. Frustrating scenario