That was my best run so far!
But I can't get how it could happen that many of my items eventually appeared corrupt. I firstly became suspicious when my Excalibur inexplicably went cursed. And after that vampire lord had turned up, I totally lost control over the situation, there was also a spellcaster who kept shooting at me with painful spells.
Some spellcasters can curse your items. Or perhaps you read a cursed spellbook when you tried to learn magic. Or prayed while your god was very angry. Or sat on a throne while unlucky.
My questions are how could I get over that water
Lots of ways.
Levitation. Float there.
Teleportation plus teleport control. Bamf there.
Teleportation plus patience. Bamf all over the map.
Water walking. Walk over the water.
Magical breathing. Walk through the water.
Beams of fire. Boil away the water and walk over the revealed floor.
Beams or blasts of cold. Freeze the water and walk over the ice.
Polymorph plus polymorph control. Any of the above.
Scrolls of earth. Push the boulders into the water and walk over them.
Wear jumping boots. Jump between islands.
Be a knight. Knight-jump between islands.
Dig a hole and jump down. Bypass the level.
and how do I train the samurai at spellcasting?
Basically, you don't. Samurai are warriors, not casters. Stick with a few level 1 utility spells. Maybe some level 2 or 3 spells once you're above XP level 20.