I can imagine two real life issues with pit traps vs seals:
1. A lot of rock shore, so you can't dig into it.
2. Seals don't venture far away from the shore, and probably don't like climbing, so a pit would probably get filled with water from below fairly quickly (especially if the soil is mostly sand rather than mostly clay).
(And my spring stranded character didn't get any seals or land animals before getting of the single tile island after 2-3 months. Must have been rather tired of roasted pike, with one bird after a month and 4 more in during the rest of the period as the only relief. Injuries healed just quickly enough to win over starvation, allowing for a first pike at a starvation of 13).