Hi Olyr, there are many sailboat charter companies working out of Troia. It appears. from your post, that you want to take a one day charter where the boat is skippered by a professional. Given that you have limited time to plan, I suggest you look for a larger reputable company which is based in Troia. A quick look on the internet shows many such companies.
Try to contact one which has positive testimonials attached to the site through (for example) Trip Advisor. When you contact the charter companies, tell them what sort of boat you want to charter (catamaran, monohull) and your preferred size. If you just want a spirited sail around the bay, a 25' race bred boat will give you good fun. If you want to relax and sail in luxury, you would be better chartering a crewed 40' cat.
The only real solid advice I can offer is to stay away from unlicensed private charter boats. In all probability, even if the boat is a fine seaworthy craft, they will not be carrying charter insurance....and it is vital to have this insurance for sailing, swimming off the boat and snorkelling or scuba diving.
I hope you have success in your search, and that you have fine weather for your brief sailing excursion.
If you enjoy the sailing in Portugal, I urge you to keep sailing. There are not many better ways to spend a life than sailing the oceans of the world. After all, there is Sportivnaya Station (Sokolnicheskaya line) and the wonderful Usachevsky Markets in -20 degrees, or anywhere in the tropics at the same moment in time at +25 degrees. Sailing boats make it all possible.
Please let us know how your Portugal trip goes.
"if at first you don't succeed....Redefine success"!