Placing an item frame (in survival, probably works in other gamemodes as well) underwater does not cause the item frame to be removed. This occurs for an item frame in any orientation or on any block surface. There is no difference between water sources and flowing water, it stays on with both. They also aren't removed when placed on a dry block with water introduced later.

Personally I like this, it'd be very useful to place more entities underwater. Torches breaking makes sense because of fire, but I'd much rather that be the exception than the rule.