random guess:

>! one of: stars/ navigation by stars /north star/ southern cross

because:

I could not be measured- there wasn't the time:
It takes a long time to chart the position of stars and know how the stars move during the year, or exactly what star positions correspond to what coordinates / bearings

Yet my knowledge was treasured to help reach friendly climes:
Navigating by the stars was the primary way of getting around on the oceans for e.g. the Magellan expedition to the Americas, the Cook expedition to Australia etc.

My ignorance - feared, for it might lead to sinking:
obviously if you didn't navigate properly at night or during fog you could hit rocks

Now I'm handily found -it's a matter of lateral thinking:
The altitude of the star can be measured using the hand stretched out in front of the eyes, with shoulders squared. Different configurations of the hand (e.g., a fist, a span, two fingers, three fingers, etc.) can be calibrated to distances (degrees) in the sky. If the navigator knows the altitude of the meridian crossing of a star at a particular latitude, and he can tell when the star is crossing the meridian, he can estimate his latitude by measuring the altitude with his hand.