Question: I have an idea that the seas of a world are red, not blue. However, the world should also be the birthplace of an intelligent alien species. Would it be scientifically possible to have a world with a red sea and where intelligent life can still develop?

Answer: 
I decided to delete my comment and answer the question.

Here's your problem:

[![enter image description here](https://i.stack.imgur.com/tlSDz.gif)](https://i.stack.imgur.com/tlSDz.gif)

This is a chart of color absorption for water. It shows that blue is reflected very well (water doesn't absorb blue). On the otherhand, it absorbs red very well (no reflection). It's exactly the opposite of what you want. But...

1. You can shift the solar color from yellow to red. This means less blue spectrum is transmitted to your world and more red. This would help (although the seas may appear more yellow than red), but they would appear dark since most of the red wavelengths are being absorbed. But, it's a blank canvas. *This is really important.*
2. We need your sun to pump out more ultraviolet. This means your aliens will be very naturally resistant to sunburns on other planets because they would have evolved natural UV protection. Why do we want this?
3. Because Chlorophyll glows red under ultraviolet light.

BUT! As with all things, you can't simply play with the dials on your planet without consquences. Many things glow under UV (a black light) and they'll be happily glowing, too. I've already mentioned the need to give your aliens natural UV protection. And playing with the sun's color means playing with the nature of vegitation. Can you get your intelligent aliens? Sure! 

But it also means you need to pay attention to the details of what else will be affected by whatever solution you choose to make your seas red. For instance, the price you pay to get glowing chlorophyll is that you may not have it since your plants may need to find a way to thrive in a UV/Red predominant energy source. Whatever you choose, take the time to think through "what can go wrong with this?" It'll actually add a lot of cool interest to your story. ("What do you mean you can't eat lettuce? It's the most neutral plant on our planet!")
The reason our own ocean is blue is because of the color of the sky. If your sky is red/orange, you might end up with a red ocean. You could also do what a couple others have said and try an alga that's red.