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.

A 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.