A little reality first :
This world's biochemistry would, by definition, be completely alien, so there's a very good chance everything could be poisonous or carry infectious organisms, at least as far as the visitors are concerned. Even breathing the atmosphere could potentially be lethal as it may carry airborne bacteria and other small organisms, never mind be toxic in some way (too much oxygen can even kill).
This works the other way as well. The visitors could introduce their alien organisms, parasites, bacteria, etc. to the world and it's inhabitants. Even in human history there are multiple examples of visitors from one place visiting an area previously isolated from them which resulted in one or both passing a disease on to the other than they had no defense against. And that's just within the same species and planet - the dangers of actually meeting completely alien life are pretty hard to estimate to quite real.
What kind of food ...?
Well as you describe it it would be very strange if the natives did not exploit the sea where it was possible. That would be the norm along the coastal areas. The areas inland where travel to the sea was not a realistic daily option would probably undertake agriculture of some sort and possibly trade with the coastal regions for e.g. dried sea foods.
However how diverse and what kinds of food would be possible is impossible to say. We have only one example of life (Earth) and from what we can tell life will develop and adapt to a wider range of conditions that even the surface and sea provide normally. There's really no way to say how your vaguely outlined planet's life might be, except that it would be surprising if it were not equally diverse and complex. What food humanoid natives might make from that is beyond guess work.
But the visitors would have to be extremely careful to manage potential biohazards from trying to eat the natives' food.
Meanwhile, any kind of plant that can't stand heat or humidity would be non-existent, which means there probably aren't any grains to produce baked goods like bread from.
Your natives would doubtless have something that fills the role of grain and bread for them. Plants grow in all sorts of extremes of heat and humidity just on Earth, why would this not be the case on your world ? If memory serves me Cassava Flour is used to make baked food stuffs in e.g. Brazil. It's gluten and grain free, AFAIK.
is fish with jam the only thing my explorers are going to find here
Fish would again be off the menu for any but the most suicidal explorer. Jams are a way to preserve fruits and while they're probably less dangerous in terms of biological infection, they'd be just as dangerous in terms of potentially toxic side effects for the visitors.
You're also neglecting all sorts of potential food stuffs (for the natives). Insects are one example. Again the wide range of what qualifies as food for humans just on Earth is limited only by personal taste and whether we've figured out a way to eat it safely. Heck we drink alcohol and that's a poison ! Likewise caffeine. If offered a drink by the natives the odds are high it would at best make the visitors ill and at worst kill them instantly.
We cook foods to eliminate most health threats in food. However just because the natives handed the visitors something cooked does not mean it is safe for the visitors. It could still contain chemicals and even bacteria and other organisms which don't affect the natives at all, but would be extremely dangerous (or lethal) to the visitors.
Stephen's Rules for Planetary Exploration :
- If it goes down to the planet it stays there, to avoid infecting the ship. That includes people.
- What goes down has to be sterile (meaning lifeless, not just clean).
- Look, don't touch. We have sensors, we use them.
Those clearly aren't TV rules !