Eberspacher do not specifically recommend the need for exhaust sealing paste at the heater exhaust connections, though of course your aim is always to create a gas tight seal throughout your boat heater's exhaust system. We always strongly recommend the use of the correct genuine Eberspacher exhaust clamps throughout for your 24mm exhaust flex, please review our weblink below.
https://www.butlertechnik.com/item/Eberspacher/or-Webasto-Heater-exhaust-clamp-for-24mm-or-22mm-exhaust-15261102-91383B/6T1
Stainless steel flex does require the extra available clamping load availble from these clamps