I don't mind the idea of sailor credo in general, but, overall, this particular idea doesn't look like too interesting and doesn't even feel like having much to do with sailing.
To give some specific feedback:
Mr_Bober wrote:Sailor Credo.
Depending on Nomad, maybe Pearl Diver. Ideally Pearl Diver should depend on Sailor.
Ok, so its either credo of 50 or 80 quests minimum. So rewards should be respective.
Mr_Bober wrote:- Constitution +25;
Boring, but thematical and fine I guess. It seems that every credo needs boring and pointless stat upgrade. I would change it to +25 Con and +25 swimming at least.
Mr_Bober wrote:- Increased swim speed (stacks);
- Better trash recovery when fishing (stacks) or shipwrecks;
- Significant damage reduction to boats when crossing into a whirlpool;
- Increased speed in shallow water;
All of these are marginal, but that is fine. Usually credo gives 2 or 3 such marginal bonuses. I say, keep last 2 as at least they are not boring repeats of previously earned rewards.
Now, the credo should have a reward or two that are actually worth getting credo for and which are, preferably, support described playstyle.
Mr_Bober wrote:- Increased speed with dugouts and rowboats;
This one could potentially be it, except it is very mind-boggling why knarrs aren't included here. Both from thematical and gameplay perspectives. If anything, I think it should be otherwise - only knarrs would be affected. Simply make it add 1 level to the top speed of any particular piloted knarr. So speed range would change from 4 - 8 interval to 5 - 8 when piloted by sailor. Or maybe even 5 - 9 would be fine and will make it relevant for, hopefully fixed in the future, nautical PvP.
The credo would probably need one more good reward and there are a lot of ideas that can fit this role. My personal proposal would be free anti-drowning "Silver for the Ferryman" effect with a significant cooldown (week or so), but I can think of plenty of other rewards.