Yes, it is more similar to the spyglass than the range finder as it "spots" and marks an enemy ship for a short duration. This Captain Mark marks the ship with lines arranged in a "+"-like way instead of boxing the ship in like the spyglass does, making it easy to differentiate the captains mark and the spot. The captain target/mark thing does not show you the ship on the map though, it is quite literally to tell your crew (I think it works for AI (not the clan) too, but I am not entirely sure if it is implemented and if yes, if it was done so correctly) which target to prioritize.
You only have to press it once, but since it disappears after a few seconds you will have to press it again, unless you trust that your crew remembers the target you want them to shoot at. Maybe if you hold the button the game will interpret it as "pressing the button continuesly the entire time" and will keep captain targeting it, but since you have to keep aiming at it and make it harder to fly your ship with less fingers it would be quite silly to do so.
It helps spotting targets too, since you can tell your crew quite literally "They are to the left" and then they see your captain target thing on them immediately so they don't have to look for the enemy and can just spot it immediately with their spyglass.
Since you need "direct visual contact" in order for it to work, it has the exact same weaknesses as the spyglass and rangefinder. Which means, when either of those tools don't work because of could logic than your captain target/mark thing won't do anything either.
Range finder was never intented to be useful. As far as I understand it was only made so gunners can teach themselves when to use lesmok rounds on a lumberjack or how high to aim with the hades when the enemy is this far away. It is purposfully made to be a completely useless tool as it only serves for gunners to practise aiming, a person that can already do that will not need a range finder. The circle the range finder creates was supposed to assist with aiming, but apparantly nobody really knows how that circle works.
