1) Nobody wore plate armour in the 18th century, you know what else happened in the 18th century? The American revolution. Saying sailors in the 18th century didn't wear plate armour is like saying "Humans don't have tails." 18th century is not medieval times at all, everybody used gunpowder
2) Nobody in plate armour trains to climb or swim. Knights often went on horseback or in heavy units where they wouldn't need to do either of those. The peasants would storm the walls while the knights stole the glory. That isn't any sort of contrived opinion that's what happened.
3) The plates are very unwieldy. You can't move anywhere near as freely as someone without. Even just the padding would make it really hard to swim (and yes they had significant padding that was very absorbent). Imagine fighting in a body fitting sleeping bag (as an extreme example). The reason this padding was there was to stop the plates from scraping skin off as the user moved. 
4) Because the armour was so bulky training wasn't about climbing or swimming, training was about stamina and learning to fight in a fridge, not dance in one as you claim. Keeping up your strength in combat while realising how to use your armour as a second shield, not about providing yourself with mobility
5) You may consider this nit-picking, but saying 'Wouldn't slow you down at all' is an exaggeration in itself so don't complain at Phoenix. Any weight will slow you down to some extent
6) Saying magic is obselete when compared to a gun is foolish. Bear in mind it depends on what lore you are using but not all ideas of magic are weak fireballs, the reason gunpowder may still have ruled out magic is because it is easier to learn and thus can be mass deployed.
And not all lore follows that casting a spell can kill you, and muskets certainly can misfire disfiguring and blinding the user for life
Not to mention magic is not restricted to fireballs, there is also healing and protection, illusion. All warfare is based on deception