"Can I get fish?"
A straight "Fish, please" should be accepted in my opinion, though others may think the level of politeness doesn't quite match up with the original Japanese phrase.
And strictly speaking, ください isn't simply "please". It comes from the respectful sonkeigo verb 下さる which means "to give (me)", so literally it translates closest to "please give me". And that's why we use を, to mark the direct object the verb is referring to.
As in さかなください? Basically the translation would be unchanged :) though it becomes slightly less formal; omitting particles is done quite often in speech, but it's a bit stricter in writing.
In Japanese, a lot of things can be omitted if they are obvious from context, and in this context of asking for something (made obvious by the nature of ください), it's clear you are asking for fish, not asking them to give it to the fish (さかなにください) or to do something by means of fish (さかなでください). In those two cases, they are unexpected interpretations so you should use the appropriate particle to clarify your intended meaning.
I think of it as "could I have" (kudasai even kinda sounds like it). Or "I'd like the fish, please" Perhaps this is a colloquially is said slightly different. Some report that they used the purse "could I get..", which should rude to me but each locality might have a slightly different accepted norm for the appropriate response. I think their translation as "fish please" is a good one, as technically, we aren't asking a question, but instead indicating a responds, or making a request. I also think that many societies have polite forums of making said request. Here, is often done as phrasing it as a question (as we aren't really telling someone to go do something). If they had asked us to translate the sentence " could i have (donde) fish", then that isa question, and would be asked differently (requiring "ka" are tree end). So, in kind, "fish, please" I believe is an accurate representation for answering an (implied) question of "what can I get for you?". Here we might say "fish, please", or "could I have the.." , whereas " kudasai" would be the/an appropriate response in Japan.
It's notreallya question - it's more like a polite way of specifying/indicating the thing that you want, when the other person is waiting for you to tell them.
Otherwise, once might also say "I'd like..." Or "I want..". It comes across slightly less rude or demanding." --- "fish please" I take as not a "literal" word-for-word translation, but instead a translation of how one society/language decides to handle an particular type of exchange. ---