Ephesians 4:8 *Wherefore he saith, When he ascended up on high, he led captivity captive, and gave gifts unto men.
9 *(Now that he ascended, what is it but that he also descended first into the lower parts of the earth?
10 *He that descended is the same also that ascended up far above all heavens, that he might fill all things.)
Notice in verse 19 it says preached to spirits in prison, the next verse goes on to describe who those spirits were and they were from long before Jesus's time on Earth. The verse above says Jesus was lowered into the lowest part of the Earth It seems to me what Jesus was referring to when He said " today you will with me" be in paradise" would have to be Abraham's bosom, a holding place for those who existed before Jesus came to the Earth and rose from the dead.
1 Peter 3:17 *For it is better, if the will of God be so, that ye suffer for well doing, than for evil doing.
18 *¶For Christ also hath once suffered for sins, the just for the unjust, that he might bring us to God, being put to death in the flesh, but quickened by the Spirit:
19 *By which also he went and preached unto the spirits in prison;
20 *Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water.
Prison means in this verse:
fulake -- pronounced: foo-lak-ay'
from 5442; a guarding or (concretely, guard), the act, the person; figuratively, the place, the condition, or (specially), the time (as a division of day or night), literally or figuratively: KJV -- cage, hold, (im-)prison(-ment), ward, watch.
See Greek No. 5442 <G5442>