Question: How should one remove the seeds from a watermelon on Shabbos?
Answer: The Gemara (Shabbos 74a) teaches that the forbidden melacha of borer involves selecting the pesoles, unwanted items, from the ochel, one’s food. The Rema (OC 319:4) writes that even if it is a lot of bother to remove the ochel, the food that one wants, from the pesoles, the waste, one cannot remove the pesoles on Shabbos.
The Shulchan Aruch (OC 319:16) is clear that the prohibition only applies to preparation of food. Eating itself, however, cannot ever be considered as a melacha. Thus, the Chazon Ish (Shabbos 54:1) writes that one must place the piece in his mouth and spit out any seeds.
The Ben Ish Chai (Beshalach 2:7) and Kaf Hachaim (OC 319:47) however, do not require this, arguing that this isn’t the normal way of eating. One should shake the melon to shake any seeds off it and pick out any remaining seeds before one eats. Similarly, R’ Binyamin Zilber (Az Nidberu 1:26) argues that if one wouldn’t normally eat that way during the week, then it isn’t considered a normal manner of eating and one doesn’t need to spit the seeds out on Shabbos.
R’ Moshe Feinstein (Igros Moshe OC 4:74 Borer 7) writes that ideally one should spit out the seeds. Where that is not feasible, such as when feeding one’s children, one should shake the melon before picking any remaining seeds out.
R’ Yehoshua Neuwirth (Shemiras Shabbos Kehilchasa 3:17; n34) writes that while it is ideal to expel the seeds out from one’s mouth or at least shake them off, if one doesn’t wish to do so, they can pick them out. As there are poskim who justify the practice of those who remove fish bones there is justification for removing the seeds by hand when necessary.In conclusion, if one is uncomfortable spitting the seeds out, one can shake each piece and remove any extra seeds. One must do so by hand and only right before they eat it.