If you can get a decent opening then the ancient trait must be the strongest since with just a dozen planets say, a couple of artifacts and crucially a crystal (which gives you base research in every planet) you already have huge amounts of research with a serious base rate per turn in every planet. (You improve the starbases by going along governance -> precursor legacy and precursor relic understanding; and later along the bottom of the colonisation tree starting with xeno artifact studies.
It also gives you a tech line just a short way into engineering, precursor traditions -> healing hulls, which gives you tactical repair early on which is v useful. (I suspect it means that the Zindelit can safely examine precursor anomalies by itself which is obviously v useful before you get bigger hulls and/or more logistics)
As for racial types aquatic is surely the best as AdamMG explains above.
So if you want a custom race it should be aquatic, ancient and another trait of your choice. which is what Publius has done. He's added vigilant and AdamMG suggested instead adding xenophobic to maximise the science: the ship building penalty could then be offset by making the xenophobes productive (+20% overall production) and militant (+20% ship production).
Personally I haven't used custom races so my favourite is the Altarians. The Drath are also ancient but their second trait War Profiteers is (much) less useful than the Altarians' Certainty which boosts cultural expansion and also gives a tech bonus each time you get a new ideological trait.