I have built two identical hostile mob farms, but only one of them can spawn mobs. The one located above ocean cannot spawn mobs. I've confirmed this by creating platforms and have observed that mobs only spawn on the part of the platform above land. I'm not sure whether this is related to chunks or to the ocean as I have only tested two areas.
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The area where you built the non-working farm is actually a large river, not an ocean. You can tell it's a river by the fact that salmon spawn there. We are tracking this issue at MCPE-161245, so I am resolving this report as a duplicate.