Plastics don't expand worth a damn - certainly not enough to warrant filling a bath with warm water while you seal it. Worry about the weight in the bath rather than the temperature of the water.
silicone moves anyhoo therefore any slight expansion of a plastic bath should not make the silicone split.
Unless you have had the bath fitted incorrectly and it likes to move of its on accord which it shouldn't.
If you are really that worried then pull a long hair out of your head or anyone else who is in the house, place hair across gap to be silicone filled, place a dot of superglue on each end, then sit in bath and see if hair breaks. Or if your really paranoid get two people to sit in the bath.
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