Hi,

I've tried many times to cast "summon boat" but it notice that: "There is no boats to summon" or "Christian try to summon boat but not success..."

Thank for your help!

Not sure of the exact details, but I think this is how it works: If you don't have Expert Water skill there's only a percentage chance (1/3 to 2/3, I think) of summoning a boat, and it needs to be already on the map.
With expert skill it always works, assuming there is room to summon the boat.

"The exact details" :) are as follows:
If a hero doesn't have Water Magic skill or has it on a Basic level - Summon Boat spell has 50% chance of success and there has to be an existing, unoccupied boat on the map.
With Advanced Water Magic skill, hero has 75% chance of successfully summoning a boat and if there are no boats on the map, one is created.
Expert Water Magic allows casting of Summon Boat which always succeeds and creates a boat, if needed, as well.

Thank PetrusOctavianus and Kerebron very much!

Now I know that it is because of the percentage chance of summoning a boat, my hero doesn't have expert water skill.