When swimming in water, or when in a boat, items held in your hand will darken. The pics I attached compare swimming to standing on land with the lighting on the items.
MC-49580
Swimming in water causes held items to "darken"
Resolved
Fixed
1
Kevin Huckins
migrated
Confirmed
Minecraft 14w08a, Minecraft 14w10b, Minecraft 14w10c, Minecraft 14w11b, Minecraft 14w17a, Minecraft 14w18a, Minecraft 14w25b, Minecraft 14w26c, Minecraft 14w28b
Minecraft 14w31a
Retrieved
Confirmed, but probably intended.
Your items are getting wet, thus darker...