The result was decided by mid-afternoon, with the visitors dismissed for less than 100.
Shaun O'Leary (24) – one of five O'Learys in the lineup – got a start, as did Mark Fraser (15) and Hadleigh O'Leary (17), who saw off 105 deliveries.
But everyone else was dismissed in single figures, with United's five bowlers sharing the honours.
The pick of them was Regan Fricker (3-24), who cleaned up the tail, while Speers (2-27) had United off to a great start by using his off-spinners to get both Marist openers by the sixth over.
United will now face Kapiti Old Boys in the final on January 8-9, after they picked up a competitive five-wicket win over Levin Old Boys at Donnelly Park.
In the minor playoffs, Burger King Red Star travelled from Masterton to get a first-innings points win over Property Brokers United, who were also dismissed for less than 100 in their first turn at bat, while still being 128 runs behind for their second innings at the close of play.
In the Premier 2 40-over competition on Saturday, Treadwell Gordon United 2nd XI maintained their unbeaten record with a comfortable 66-run win over Kaitoke Knight Riders, as the games were moved to the Springvale fields.
Tech Old Boys kept pace with the leaders after a 38-run win over Wicket Warriors Whanganui, while the Wanganui Renegades locked in the third spot on the table with a five-wicket victory over Matt Burke Engineering Marton Saracens.
Results, December 18-19
Coastal Challenge
United CC 297 (K Dhiman 70, J McKenzie 42, G Singh 37no, R Speers 32, S Jonas 27, S Payne 24; F Kinnerley 4-60) bt Wanganui Vet Services Marist 96 (S O'Leary 24; R Fricker 3-24, S Payne 2-10, S Patel 2-14, R Speers 2-27) on first innings.
Levin Old Boys 147 (M Good 27, R Taylor 27, J Pinfold 23, B Te Tomo 21; T Harris 4-27, C Andrews 3-28, Z Benton 3-52) and 169-8 dec (B Te Tomo 58, A Simpson 32, J Pinfold 30; A Halbert 6-24) lost to Kapiti Old Boys 258 (M Harrison 46, J Miles 43, A Halbert 40, S Paterson 33no, D Browne 33, T Harris 28; X Sakhalkar 3-45, R Taylor 2-36, A Simpson 2-37) and 61-5 (R Hickey 23no, J Miles 20; D Kennett 2-23) by five wickets.
Burger King Red Star 172 (S Hooker 55, N Elliot 25, A Singh 23; L Hoekstra 3-32, C Meredith 2-14) and 129-4 dec (J Jonas 63, S Hooker 37, P Sigvertsen 20; A Penn 2-15) bt Property Brokers United 78 (A Penn 16; S Hook 5-20, J Church 2-13, P Sigvertsen 2-33) and 96-4 (T Lance 42, A Penn 26, C Hobbs 20; S Hook 2-18) on first innings.
Weraroa CC vs Paraparaumu Medical Centre CC unknown.
P2 40
Matt Burke Engineering Marton Saracens 132-9 (D Rayner 61; R Moore 3-23, R Donaldson 2-18) lost to Wanganui Renegades 133-5 (J Trillo 61; S Singh 3-40) by five wickets.
Tech Old Boys 123 (Q Mailman 40, C Greene 30; S Sreekumar 3-12, R Sabari 2-14, L Varghese 2-18, A Coelho 2-22) bt Wicket Warriors Whanganui 85 (V Chacko 15, A Coelho 15; A Tipper 5-26, J Healy 2-16, D Maraki 2-17) by 38 runs.
Treadwell Gordon United 2nd XI 162 (K Watson 48, K Bremer 37; M Slade 3-47, F Edwards 2-11, M Tongotea 2-19, J Malipaard 2-21) bt Kaitoke Knight Riders 96 (Z Payne-Potaka 31, D Whetton 23no; K Bremer 5-23, K Watson 2-19, L Singh 2-26) by 66 runs.
Marist Bs vs Wanganui United 3rds unknown.
Whanganui Collegiate Senior Black vs Whanganui Collegiate Senior Blue unknown.