While this may be true, it also has urban myth status - and gets retold regularly with many varients. Popular ones include the offending implement being a spear, and the death being the reason for the disbanding of the Second boat club. All have the event take place during some bumps. Perhaps someone could go to StJohnsCollege's and read the "History of the Lady Margaret Boat Club" (3 vols) which is in their library.
Believed untrue by LMBC members. Supposed true story is that the original St John's boat club existed only for people who went to Eton or Harrow; a less elitist LMBC was formed somewhat later.
Sally asked the dean of college about this, and was told that John's had never killed a Trinity cox, merely a Trinity Hall cox. Thus explaining the black of the Tit Hall boaties as well as the scarlet of LMBC. Whatever the myth, it is true that John's are not allowed to register boats on the cam - Sally believes their vast fleet of punts are all taxed in the name of Magdalene.
A possible origin for the Myth is a real event in 1888 Lent bumps although this has nothing to do with Johns:
In the first two days of the Lent races, 1888, T.B.C. went up two places. The third day was the occasion of a sad tragedy. Clare bumped Queens', and drew into the bank by Grassy. Behind these boats was the Trinity Hall third boat. This, instead of rounding First Post Corner, ran, by some mishap, across the river, and the nose of the boat struck number 4 in the Clare boat just over his heart, killing him on the spot. The further races were at once stopped. Since this dreadful incident small india-rubber knobs have been fixed on the bows of all the racing boats. At the end of the races, the first six boats were Jesus, T.B.C., Corpus, Lady Margaret, Caius, and T.B.C. 2nd; the T.B.C. boats finishing 2nd, 6th, 16th, and 22nd, and the Third Trinity boat 10th.