Is it true that U-boat crews in WWII had the highest loss rate of any unit in military history?

Hmm... Arguable. Also depends on how one defines 'loss rate.' For example, the Japanese military had absolutely heinous loss rates, but that was also because they had this habit of not surrendering very much. For example, they started the Battle of Tarawa with some 2,600 troops (and another 2,000 support personnel), 17 soliders and about 120 laborers survived, so about 97% lost from those two units. (Third Special Base Defence Force, Seventh Sasebo Naval Landing Force)

I believe the figure generally thrown about for U-Boat crews is 70-75% were killed.

For the US, you'll often hear that Bomber crewmen suffered the most losses, Don Miller's book "Eight Air Force: The american bomber crews in Britain" showing a rate of about 34% KIA.

But, if you drill down a bit more... Adjutant General figures for WW2 show that of all the Army's riflemen deployed overseas, 18% were killed. (If you're curious, 3% for tank crews). However, if you drill down into -units-, you see that the numbers get more complicated. 4th Infantry Division, for example, had one of the highest overall casualty rates (252% or so), but that includes wounded. The "Killed" rate was something more reasonable, about 25-30% as I recall. But that's out of the entire division's authorised strength regardless of how many people cycled through over time, and also and not everyone is a rifleman. If you look at the number of -riflemen- killed in 4th ID, and I continually kick myself for apparently not photographing the document I saw in the archives, the figure was something ridiculous like 300% casualties.

If this helps the maths make sense: Let's say a unit consists of three trigger pullers, and seven support folks. One of the trigger pullers gets killed. The unit has suffered a 10% loss, but the rifle team has suffered a 33% loss.

As a result, much as not everyone in the 8th Air Force was a bomber crewman, and not everyone in an Infantry division is a rifleman, one must ask if counting only U-Boat sailors as a unit is really comparing apples to apples.

What is beyond doubt is that being a U-Boat crewman meant that statistically you had a very good chance of getting killed. As would being a rifleman in 4th ID. Or a crewman in RAF Bomber Command (About 44% as I recall). And so on. Unfortunately, true drill-down figures to make apples-to-apples comparisons such as 'most dangerous job' are actually quite hard to find.