Don't even consider a telescope. Your best VALUE on a boat is a good pair of standard 7x50 binoculars with built-in bearing compass.
You want 7x50 because (as others have stated) 7x magnification is the best compromise between maximum magnification and manageable "shake" while on a moving platform, and a wide field of view is also desired on a boat (for spotting buoys etc.). The reason you want a 50mm objective lens is for maximum light gathering at night, and while the larger objective lens makes the unit heavier, you will find you can see much more at night with 50mm binos than with the naked eye (or with compact "opera glasses"). Great for spotting buoys etc.
An integrated bearing compass is another very useful feature: By taking a bearing (compass directional line, in degrees) to an object, then another a few minutes later -- you can use it as a quick check to know if you are on a collision
course with another boat or ship... it the bearing remains constant you are on a collision
course, but if it changes over time you are safe. Of course you can also use it to plot your position on charts
using triangulation relative to fixed charted references
, as a fall-back to GPS navigation
I strongly recommend you spend the money
for a quality brand, such as Nikon, Canon, Steiner, Fujinon and avoid low-end "house brands". (If the normal market price
for the brand/model is cheaper it is certainly
of lower quality.) The difference in optics is significant when you want to read details, but even more obvious will be the precision of the alignment between the two sides in binoculars. Even slightly misaligned binos can drive you to distraction and eye strain to a point where you won't feel comfortable using them. Don't cheap-out! A quality brand will last many years, so think of them as an investment.
We have Nikon 7x50 with integral bearing compass and keep it handy mostly for the bearing compass function and for night vision. We also have the Fujinon 14x40 image stabilized and they are so good it is amazing!
(Expensive though.) If we want more detail or want to hail a boat by name, we pull out the Fujinons.
Even though they are more powerful, the image stabilized Fujinon is definitely less useful than the 7x50 standard binos for general navigation