TO answer your question about underground water without an aquifer in Dwarf Fortress. You will need to dig a cistern and Designate the cistern as a pond under zones. Your dwarves will fill it with buckets so make sure you have made a few. this may require an appointed manager. Or the harder but more FUN way id to divert the river by first tunneling underground to just below and next to a river on ground level. Be careful water can flow diagonally between tiles so make sure you have a full "L" worth of stone/dirt your dwarves should stop automatically if the reach a damp tile. But they will dig any way if you tell them to dig it again.

more simply would be to embark with a light aquifer and dig a cistern into it it saving often and planning carefully light aquifers can be FUN but fill mined out tiles at a fair and managable rate so long as you don't panic.