The Navy uses highly enriched uranium for its reactors, something like 70-80% enrichment. This is a non starter for civilian use, on account of proliferation concerns. That, and the enrichment requirements drive up fuel costs.
What does the enrichment of uranium have to do with humans working in proximity? In both low-enriched land based plants and in marine nuclear power plants, the radioactive materials are contained in the pressure vessel and inner cooling loop.
So in other words: a non-naval SMR which doesn't use HEU is going to be substantially larger - which would make it substantially more expensive, and therefore not representative of civilian SMRs?
Who said anything about expense? Why does "bigger" equal "more expensive?" Lead, concrete, etc. are cheap on land, but volume is a precious resource on a ship. HEU isn't the only reactor fuel out there, either.
And of course, all non-naval reactors are naturally going to be larger: they aren't surrounded by a practically-infinite fluid heatsink.
Land-based deployments don't have this constraint.