Is there any magical equipment that allows Armies to cross water without a boat. It has happened twice to me now. I first assumed it was a bug.
But when it happened a second time, I began to wonder if it might be a piece of equipment. When I say cross water, I mean I saw an Army walk off the shore into the Dark of Unexplored Sea.
Thanks

Yes. There is a level 4 spell 'Water Walk'. If enemy hero got it then he/she can walk over water after casting it. But he/she should land on the solid ground at the end of the turn.

P.S. By the way, lvl 5 spell 'Fly' (and corresponding artifact 'Angel Wings') can be used almost the same way also.

There you go. Thanks.

And there are artifacts that give you both Water Walk (Boots of Levitation) and Fly (Angel Wings).

Also Tome of Water Magic gives Water Walk and Tome of Air Magic gives Fly and Dimension Door (not mentioned before, it can be used to cross water, if it's not too wide), as they grant all water and air spells, respectively. Spellbinder's Hat gives all 5th-level spells, so Fly and Dimension Door are included. And, obviously, there can be a Spell Scroll containing one of beforementioned spells.