Hi, you should write c'è (ci + è).
The Italian sentence corresponds to the English version Who else is there on the boat?, which is usually replaced to the only "is" in English, but in my language it' s not possible to omit ci because it indicates the presence of something / somenone, it's like a subject. I add you can translate this sentence and in particular "else" with Chi altri c'è sulla barca? too, when you need to be more formal (but I don't know if Duolingo will accept it).
As a native English speaker, 'on' the boat, plane, train, bus, etc. is more correct. In the boat would be much more limited, and refer mostly to small kinds of boats. 'He is in the row boat on the lake' is ok, but 'on' is always used for larger transportation vehicles. One is always 'in' a car, though.
Anything smaller than a yacht, i would say 'in' - so rowing boats, canoes, kayaks, dinghies… but yachts and anything larger i would definitely say 'on', and that's what i would understand from the word 'boat' in english. i don't know what the specific connotations of the word 'barca' are for italians though
English distinguishes between in and on, but not consistently. Generally we are IN enclosed things but ON flat ones. We say on a (flat) plate, but in a bowl - even though it is uncovered. ON a horse, bicycle - and abstract things like 'trial', 'holiday', 'speaking terms'. IN a family, club. IN a British team, ON an American team. With seagoing vessels and aircraft, it can be a matter of size, shape, or just regional convention. I prefer IN a boat, canoe, helicopter. IN a ship unless ON the top deck. Majority use may be ON an aeroplane, train or bus even though they are all enclosed. But no-one would ever be misunderstood for making different IN/ON judgements from others. We also use IN/ON/AT for times and places where the choice can suggest the degree of precision. [IN an hour, AT noon, ON Friday, IN April; IN town, AT the crossroads]. There are regional differences: ON the weekend in the US; AT the weekend in the UK. 'On Christmas' is weird to British ears, but 'on Christmas DAY' is not.