You get the full 1.13 experience, including shipwrecks, coral reefs, dolphins, tridents, and the revamped swimming mechanics.
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Yes, to a limited extent. Eaglercraft 1.8.8 supports standard Minecraft 1.8 resource packs loaded as .zip files. There are also community projects like EaglerForge that add a Plugin API and custom modding capabilities to Eaglercraft.