What happens to inventory when you die under ice

Assuming vanilla Minecraft, items only remain on the ground for 5 minutes without any mods. If it took you longer than 5 minutes to get to your place of death, your inventory is gone, sorry.

Note that there are gravestone or gravechest mods out there that will spawn a chest with your items or a gravestone that will last until you break it to get your items back.

The location of your items depends on many factors. One factor is the items' "density". Some items will float and others will sink. Another factor to consider is bubble columns, as they will cause all (dropped) items to rise or sink, depending on the bubble column. An additional factor includes dolphins and other players, as they can move items. So, if the items sink, they'll be at the floor of the body of water. If they float, they will be waiting just below the ice you died under.