If I choose Profession (herbalist) for my PC, would that have any use besides flavor? And if my friend choose Profession (fisher) would that be good for an aquatic PC? And have a use?
Ask your DM beforehand, but the general answer is no. Profession skills can be used untrained and grant no synergy bonuses, so if you ever need them just find some way of buffing the skill check (I like to use magic) and roll away.
SRD said:Profession (Wis; Trained Only)
Untrained
Untrained laborers and assistants (that is, characters without any ranks in Profession) earn an average of 1 silver piece per day.
Oh, I must have been thinking about Craft then.
Personally, if one skill were to be trained only out of the two, Craft seems more natural. Never understood why being a guard is any harder than crafting a sword.
Craft is untrained because, despite not being a woodworker or a carpenter, I can still make shelves and wooden shields.
I can also whittle a spoon, do basic pottery, make a lean-to, and fashion a spear (out of metal).
I am a modern, largely-city dwelling individual - though, admittedly, I was in Boy Scouts for many years.
If I, as such a person, can handle basic crafting skills, how much more so can the average D&D-world's inhabitant, who generall has to rely on their own abilities, rather than the corner store, for such essentials?
When it comes right down to it, I've always viewed "untrained-possible" skills as falling into one of two categories:
1) This is a key function of being alive (Perception, Hide, etc.)
2) This is something in which everyone receives entry-level training (Craft, etc.)
Generally, Profession skills are as useful as your DM lets them be.
For Prof (Herbalist), I'd let you sub in a Profession check in place of a Heal check in many circumstances, and probably also give you a synergy bonus to Heal and Craft (Alchemy) checks (and let you make any herb-based alchemical stuff untrained, regardless fo whether or not you were a spellcaster). I'd also let you sub it in in place of certain Knowledge (Nature) checks.
I'd do similar things for Prof (Fisher), with it subbing in for Survival, mostly, or various Knowledge skills.
I admit, though, that I tend to make those skills more broadly applicable than others would. What would you like to get out of those skills?
And, finally, welcome to ENWorld!