There are different quality travertine pavers. The higher quality ones are sanded down slightly to reduce the amount of holes and provide a smoother finish.
I designed two pool projects in particular last year. On one of them the Andes Collection pavers (from Peru) were used.....very few holes. On the other, anothe company's travertine pavers were used. (I did not suggest this company). The pavers had lots of holes. In addition, I happened to see it after a rain, and as you mentioned, the holes were filled with water.
Regardless, I do not think the holes enlarge over time. The long term durability is excellent....the Coliseum in Rome was made from travertine! Travertine pavers do extremely well in freeze-thaw conditions. Be sure to dry lay them.
I do not know about salt water pools, but if you call the Andes Collection, they may give you more info. Also, they will send you samples. I am not familiar with the pavers from Mexico.
If you have any further questions, just let me know.
