First let me introduce me : Old gran'ma (70+), Gamer for very long, who decided to try this game, and so far is more addicted than she thought. Though i will play it in an easy mode, as i am not able to do end game or group.
So, now i will ask, in desorder, as the questions will come (i spend a lot of hours reading guide and watching youtube but i did not find my answers) :

- Fishing and boats : After building a wharf and been totally lost for the follow (and dead after the first quest asking me to travel to an island), i decided to use my Loyalty and buy an epheria cog, may be an error as it seems that this boat will vanish after some time ? and can not be upgraded ? Right or changed ?

- Mounts : is there a way to remember where you left your horse ? and on which character,  or the best is to use paper and pen to no it ? (actually i keep running as i forget where are my horses) :P

- Houses : i bought a residence, that allow me to craft with the good tools, i also bought another house for Storage. But i notice that i can only put the storage boxes into the residence and not into the storage house ? what did i do wrong ?

- Clan Guild : i know i did something wrong but can not guess what ... i went to the npc, and choose what i thought guild but in fact that was a clan, so i tried to upgrade but that did not work so i disbanded and went to get a guild ..... to get only one message saying that the guild was still in formation .... so i am with no clan and no guild :P

if you know what i did wrong, i would be welcome to learn to avoid that in the futur

For the laughs, i lost about 60 fishs .... because i did not realised there was a timer on those :P so i am back to fish and this time to drie them so i can cook them

ok that is all. I go back to the game, and to the fantastic fights so different of what i could expect. So far i tested Maegu, Ranger, and Woosa and i love the 3

Thank you all

Hey, and welcome!

- Yes, cogs are limited lifetime. Once their durability is gone you discard them.  Unlimited lifetime/repairable ships are Bartali sailboats, Epheria sailboats and frigates, caravels and galeasses, and carracks.  You can get a Bartali sailboat in Velia, Dancing Marlin Inn, Proix NPC https://grumpygreen.cricket/bdo-bartali-sailboat-great-ocean-ship/

- Mounts: To get a path to your horse, right-click the horse icon at the upper left, below your level number.  I don't believe you need a trainer's flute or calling horn to do that.  You can then use T to auto-run to your mount, or open the map (M) to see where the path leads.  Your mount also has an icon on the map.  You can call your mount to come to you from various distances depending on the flute or horn that you have on that character.  There is also a setting (General Settings > Quality of Life > Auto-call Mount after Quest Complete) to do that automatically after some (not all) quests.

- Residences / storage: no, you are doing things correctly.  The storage unit that you place in a residence alows you to access storage from there without maids.  The storage that you get from purshasing houses adds slots to the storage capacity in the town that the house belongs to.

- Guild / clans: Someone else will have to answer that one : )  You can look at online guides.

- Fish: Yes, most fresh caught fish expire in 24 hours (whether you are online or not).  You can dry them to use later in cooking, or sell, or other things.  Or you can trade fresh fish to trade manager NPCs, or imperial fishing delivery NPCs.  You can get better prices for trading farther away.  You might also want to get the Miraculous Fish Tank, which keeps fish fresh five times longer: https://www.blackdesertfoundry.com/miraculous-fish-tank-guide/

Good luck : )

Thank you very much for the fast and great answers :) much appreciated

You need to join a lifeskill guild that is friendly, first. Don't make your own guild after watching youtube videos and getting questions answered. That is like. Like. Someone watched your life as a grandma in a film. And then decided that they can act your part perfectly and replace you. Sort of an eh example but my point is that you need to do the doing that needs to be done before you make a guild and recruit people.

Reason for that is. First, recruitment in this game isn't easy. You'll have a hard time recruiting people to begin with. Second, people don't hang around long enough. You have to be able to answer questions and create content/activities for them. You're still learning, how will you do that unless you overexert yourself. Finally, three. No matter what guild you start. Its going to have just you and slowly expand. Rather than that, just join an active guild with lots of friendly people. You get to communicate, make friends without lifting a finger and also get your questions answered right away. 

Disband that guild you created. Press G, pick Lifeskill tab and then find one from there! There's a lot of friendly ones out there but there's also some really toxic ones. Just pick and start out with a friendly one first and take it from there. Usually the lifeskill ones are the more friendly ones. PVE sometimes tend to be decent too. Avoid the PVP ones atleast in the beginning while you're learning. Most of the toxic crowd's in those and the PVE ones. Not to say that there aren't toxic ones in lifeskill or vice versa but they're usually less in number.