I recently started to read 'Made in Abyss' and unfortunately is doesn't contain any furigana, which sometimes makes it hard for me to understand. So I came across the word 電報船, which refers to a mail ship, but since rendaku is a thing in japanese, the ふね might become ぶね. Or せん? Does anybody now how to pronounce the word properly (and why)?
Looks like 電報船 is a made-up term that is unique to this manga, but since 電報 is an on-on compound (i.e., a kango), people usually expect 船 to be read せん, too. As this answer explains, せん is an on-reading and thus will not be voiced into ぜん.
- On + せん: 連絡船【れんらくせん】, 貨物船【かもつせん】, 宇宙船【うちゅうせん】
- Kun + ぶね: 屋形船【やかたぶね】, 箱船【はこぶね】, 乗合船【のりあいぶね】
That said, since this is a made-up term, it may have an unpredictable unique reading, so ultimately you have to look for the official reading. I found a seiyu read the term actually as でんぽうせん in this radio archive.
This word does not appear in the standard dictionaries Kojien (広辞苑第六版), Daijisen (大辞泉), Daijiren (三省堂 スーパー大辞林), Shinmeikai (新明解国語辞典), nor in several online dictionaries I searched. It could be an extremely rare word, but it's also possible the word is a nonce which appears only in that source text. If it is a coined term, there wouldn't be an 'official' reading, and is probably contingent on the author's choice.
However, I think it would be either でんぽうせん or でんぽうぶね. From a linguistic point of view, the other option, でんぽうふね, is unlikely because the second word in the compound (ふね) is not a voiced obstruent. According to Tsujimura (An Introduction to Japanese Linguistics), only in the case of voiced obstruents is rendaku blocked in the second part of compound words.