There are many types of fish and also many ways they propel themselves through the water.
Unfortunately these techniques don't lend themselves readily to walking on water. Various species of amphibious fish have large solved these problems by:
- Evolving extended gill plates and pushing with fins and tail
- Snake-like lateral undulation
If this is an advanced society with arms able to manipulate the environment, one of the necessary requirements may be the need to use your arms in any manner you see fit whilst walking, therefore the most likely form of movement is a Snake-like lateral undulation.
For this undulation to work you need purchase on the floor - it cannot be completely smooth. It is well documented a serpentine motion is useless on smooth surfaces.
Therefore we are looking at a rough material - perhaps a porous limestone, where the motion can be easily attained with reduced effort. Ease of 'walking' would be a desirable trait for any floor.
However, having said that, human society has demonstrated people don't always make a choice solely on 'comfort' or utility, but also on other factors such as maintenance, cost, style and culture. Therefore think of all materials that are rough but with different attributes, depending on their culture or technologically advanced capabilities, such as:
- porous limestone - this may be the most 'culturally accepted' as limestone is common in certain coastal areas
- slate flooring, grouted
- Rough slip-resistant tiles - ceramic or porcelain
- Waterproof easy-to-clean slip resistant sheet vinyl flooring, if they have plenty of oil based product expertise
- Rough etched concrete, troweled or broom finished, if they have access to cement products
- Exposed aggregate concrete, especially if shells and other sea floor materials are used as aggregate, could be a good cultural mix with utility.