Searching this forum, I've found that water tiles in the outdoor map for random and LT encounters block movement, thus severely restricting the giant monsters (spiders, giant, manticores, dragons, etc.) Is it correct? But how is this even possible considering that many encounters include these giant monsters? How can they move through the map if water impedes their movement?

Yes, it's correct. It's just one more of the many ways large monsters suck.