There are concrete ships. Often these are barges, though traditional sail and powerboat designs are found (news to me as I wrote this comment, see Wikipedia link below).

I don't know how well the Roman formula would hold up, but traditional concrete tends to spall badly in constant exposure to salt water. Reinforced concrete suffers from rusting of the steel rebar segments.

Under dynamic stress, I doubt you'd get lifetimes of wear. Though several decades should be reasonably viable.

https://en.wikipedia.org/wiki/Concrete_ship

I thought I read recently that the 'secret' to Roman Cement - was that they were using volcanic ash, which changed the composition and resulted in a better ingredient.

https://www.nature.com/news/seawater-is-the-secret-to-long-l...

Yes and no. The linked article explains that the 'secret' was ash from Mt Vesuvius which added silicates. But the Romans had another cement called "Maritime Cement" which is mentioned in the second article (the follow up article that I linked) which says,

"There does seem to be one particular application in which the Romans had a formula we would like to rediscover: maritime concrete, exposed to seawater. Normally, concrete in a pier or harbor erodes over time, both due to the salts and other minerals in the seawater, and due to the abrasion of sand and silt. However, some Roman marine structures have survived for 2,000 years. Geologists studying Roman concrete have recently discovered (2017) that the particular concrete formula used in those structures has a beneficial interaction with seawater, creating new crystals within the cementing matrix that actually increase strength. DARPA just announced that “Unlocking the Secrets of Roman Concrete” is one of many topics for exploration under a research awards program."

Is it substantially better than modern concrete with seawater-proof reinforcement? Stainless steel rebar, fiberglass rebar, and basalt rebar all exist. The latter sounds quite promising if you believe its manufacturers’ claims.

(A major cause of failure of concrete near or in the sea is that the rebar corrodes and expands, cause the concrete to break.)

Yes it is substantially better because it grows in sea water. Which is to say that it is self healing with respect to erosion. They give examples of roman maritime concrete that is over 2000 years old and still functional. No existing concrete does this (hence the DARPA call for research topics).

It is also why a concrete hulled boat using this stuff would not suffer hull erosion like the existing cement fleet did during world war II and afterwards.

It isn't clear to me (but I've not found a good paper on Roman "maritime" concrete either :-)) how it interacts with barnacles and other marine life.

Very recently indeed, this is mentioned in the linked 2018 article. I assume there is some other secret sauce or it wouldn't be so secret.

The fact that they were using volcanic ash is nothing new. I mean, Vitruvius basically wrote down that it was volcanic ash.

The specific chemistry of how the specific silicates interact with seawater and how that forms new compounds is a new discovery.

This is all explained in the books of Joseph Davidovits.

Thanks for reminding me of trips to See the Atlantus at Cape May when I was a kid.... :)

https://en.wikipedia.org/wiki/SS_Atlantus

(and horseshoe crabs and Cape May diamonds and ...)