There are black spots in the water when I set reflections to ultra on my iPadOS for Minecraft Bedrock Edition. I am not using beta and preview, I am using experimental versions on main bedrock edition. I expected to see reflections of the land areas, but I got these unexpected lag spikes, and I saw these black spots that seemed to be causing the lag.
Please do not choose unreleased versions as affected (eg Future Update). You do not have access to any of these. Please choose the actual version that is shown on the first Minecraft screen.