This answer is somewhat of an addition to the other answers posted rather than a completely independent one.
I think you're asking two different questions here.
The first is the title question, what magic aids sailing the most and that's the one people have mostly answered and answered very effectively. But in the body of your question you say that this race is regarded as the best sailors around the world and their magic must play a role in that, but what role?
The reason these questions are different is that the magic which is best for sailing won't necessarily make your race known for being the best sailors.
For example:
A couple of answers have been the ability to grow fresh fruit and by implication fresh veg and potentially any plants on the ships. While on ships this means no scurvy on land this means your race just has an inexhaustible and completely reliable supply of food and drinking water, just juice the fruit, meaning their not the best sailors they're just the best. Their forts can't be taken by siege only by assault, their armies don't need to disband for the harvest as they don't have a harvest time and depending on how quickly the plants are grown their armies may not even need supply lines. Their style of warfare would probably just be to send raiding parties in to burn everyone else's crops and then when the inevitable famine happens just supply food in exchange for capitulation. Areas like deserts and mountains which are typically really inhospitable because its so hard to get food there suddenly become very desirable as that lack of food isn't an issue for them but is for their enemies.
Controlling the wind is useful on ships as it means your not beholden to the trade winds and you can't be becalmed. But on land it means you have an inexhaustible supply of power. For most of history if you needed large amounts of reliable constant power you either needed huge teams of people/animals working in shifts or a waterwheel both of which have drawbacks. People and animals can be anywhere but need a large infrastructure to feed and house them while waterwheels need a river strong enough to power it. Windmills where known about and used but wind power is notoriously unreliable and like waterwheels they can't be just placed anywhere they need constant wind to be useful. If you can control the wind though then that's everywhere. As an example of what they could do with it look up Sinhalese monsoon furnaces, people on Sri Lanka where using monsoon winds to make high quality steel in large amounts as early as 700ad and maybe as early as 300bc, so if your race can control the wind that's year long production of good steel centuries before anyone else. 164
You could also use it to power sail driven chariots for communication or trade or even war, historically horses were only really used by the rich because they're very expensive to breed and maintain, sails are not. Or airships and hot air balloons, not hard to build and could be really useful for reconnaissance but the fickle nature of the wind makes them unreliable.
Controlling the sea with hydrokinesis is a bit a different and might be viable as it's not a huge help on land. River trade would be greatly enhanced but you couldn't supercharge waterwheels really as rivers only have so much water in them. The main problem with it is that if we're in a navel battle and I can literally control the sea in any meaningful way then I've won the battle. Which in turn means that this race won't be regarded as the best sailors, they'll be the only sailors. Everyone else will have to hire this race's ships or hire members of this race to crew their ships because if they don't their ships will at best be so out competed they'll be uneconomical or at worst they'll just get sunk.
Precognition which you mention yourself is also useful at sea but also super useful on land, military campaigns come to mind in particular. If you know what the enemy are planning maybe even before they know themselves, you will almost certainly dominate the battlefield. Similarly buffing the sailors while good at sea would also be really good in an army or just for your workers.
So I would suggest that the best magic would be something like the ability to tell the time once a day as mentioned in <PERSON>'s answer. Really useful specifically at sea for navigation but not for much else. However it wouldn't explain why they evolved a sea faring culture to start with.
The ability to breath underwater however would. As well as the advantages mentioned in <PERSON>'s answer it would also provide an incentive/justification for this race to live near the sea. Which in turn would mean they would evolve a sea faring culture and so have the most experience with ships and the sea and that could be why they are regarded as the best sailors. 537