Steel reinforcement is added to concrete to control cracking to handle tension. All normal concrete cracks in tension (even microscopically) and the steel grabs hold and prevents cracks from enlarging and picks up the tension and spreads it through the length of the concrete.
But steel only comes into play AFTER the crack and only for tension.
A sphereical shape has very little tension but can have massive compression, not to mention a floating concrete ball in the water would be equally supported on all sides and any excess pressure on any given side from wave pressure would be equally distributed through the entire shell.
Back to concrete, the PVA fiver additive, they say can totally replace steel in bridge span members when used in high enough quantity as the PVA actually forms a chemical bond to the concrete that is stronger than concrete itself where rebar forms no real bonding and relys on knurling of the steel to make bumps that cannot pull through easilly.
From looking at some of the data you can see a thin slab taking 30 degree bends before failure, normal concrete will not take any 0 bending.
PVA does not rust, steel in concrete rusts dut to the fact all concrete micro cracks and lets moisture in eventually (depending on how well it is sealed) and all settl eventually will rust and when rusting it expands and cracks the concrete from around it.
The Romans had no steel reinforcament in thier concrete and thier concrete still lasts today… We have bridges falling down all around us due to the steel rusting in the concrete causing it to fail.
Nylon and fiberglass slivers do not bond with the concrete and are not structureal, also being less in weitht when mixing have a tendancy to float to the surface, another problem PVS does not have, it blends with the concrete.
A steel trowel also makes a smooth finish, which reduces drag. One issue being that it makes it hard for paint to stick, though I think that would also imply that it would be hard for clams and other marine life to stick also, and easier to clean them off.
The paints they tried, while some of them such as vinyl and heavy rubber do work, though require a different surface preparation (plastic or wood float), and they may have different thermal properties, such as when they expand-contract.
So it seems that having a bare concrete hull finished with steel trowel may be one of the best options, likely I’ll have to test it out on a small model, see how it works.