Craig Dodd, 30, had denied importing 253 grams of heroin, worth up to £150,000 by jet-ski on 15 February last year, but the Royal Court found him guilty of the offence by a 7-2 majority.
Dodd, and Oliver Butler, 35, from Jersey, had both denied landing 7.65kg of cannabis resin at Bec du Nez, by rigid inflatable boat, during the night of 13 September last year. Both were found guilty by 8-1 majority verdicts.
The cannabis was worth up to £191,000 and there was enough to make 30,000 cigarettes.
Jurats took nearly 90 minutes to reach their decisions.